No True Scotsman is an intentionallogical fallacy which involves the act of setting up standards for a particular scenario, then redefining those same standards in order to exclude a particular outcome.
The Trope Namer and prime example of this sort of behavior is a hypothetical scenario (first told by British philosopher Antony Flew in his 1975 book Thinking About Thinking) in which a Scotsman reads about a horrible crime in the newspaper that takes place in the English town of Brighton and smugly thinks to himself, "No Scotsman would ever do such a thing." Something much worse happens in nearby Aberdeen and is reported on the next day. Rather than admit that he's wrong, he instead thinks, "No true Scotsman would ever do such a thing." In this case, he is going from "someone who lives in Scotland" to "someone that meets my standard of Scottish behavior."
A similar way of illustrating the point:
Angus: No Scotsman puts sugar in his porridge! Bonnie: But my uncle Scotty MacScotscot does just that! Angus: Weel, then he's no' a true Scotsman.
This is very common within subcultures. Works or creators are discredited as not part of the genre due to not living up to arbitrary standards (or just being popular). Often this is followed by examples of what are considered real examples of the genre (see also Pretender Diss). By extension, you aren't considered a real fan of the genre if you don't know of these works. Sometimes, when dealing with a Dead Horse Genre or another sufficiently ghettoized field, the fallacy is used to try and distance a well-liked entry from it.
In real life, it's most commonly found in arguments about politics, race, nationality, or religion, usually when it comes to perceived stereotypes that something negative "can only be done" in a specific region or group of people (especially The Rival) and not the accuser's own group; with of course ignoring the fact that it can.
Essentially a form of Begging The Question, in that, to accept the argument that No True Scotsman would do X, one must accept that the definition of "True Scotsman" includes "would not do X."
Cultural Cringe, Cultural Rebel, and Boomerang Bigot are this fallacy being done on one's own culture. note Using the above example, in this scenario, the Scotsman would proclaim "No true Englishman would ever do such a thing." instead.
If the group being referred to has clearly-defined or generally-accepted membership standards that exclude the counter-example. For instance if a statement is made about "Eagle Scouts", and a rebuttal is offered concerning "Boy Scouts", pointing out that "Not all Boy Scouts are Eagle Scouts" is not No True Scotsman.
If the group being referred to has specific, objective guidelines of behavior as a member of said group. "No clean cop would take a bribe" is not fallacious, because a clean cop, by definition, doesn't take bribes. Similarly, if a religion has as two of its main precepts, "Do not drink alcohol on Friday" and "Believe that specific Book Y is absolutely true," then someone who drinks on Fridays and denies Book Y isn't really part of it.
If the action axiomatically disqualifies one from inclusion in the group. For example, "No right-handed person predominantly uses their left hand" is not fallacious because right-handed people are defined as those who predominantly use their right hand. Someone who is calling themselves "right-handed" but predominantly uses their left hand either isn't telling the truth or doesn't understand the distinction between "right-handed" and "left-handed" people.
If the term is redefined because it is susceptible to multiple interpretations, and there was legitimate confusion about which was being used. This would be sloppy, but not necessarily fallacious. It could be clarified by observing, "You Keep Using That Word..."
Kazemakase Tsukikage Ran: The samurai in the 11th episode; the only 'true' samurai in their opinion are samurai that think like them. They talk about honor in one scene, for instance, they refuse to sneak attack and decide on a time and place of neutral favor. Then they attack him three to one, even when he reveals himself to be unarmed. Everyone who doesn't act like this is a "coward" or a "maggot".
In One Piece, there's a great deal of Fantastic Racism between humanity and fishmen. Arlong split off from the Sun Pirates and formed his own crew because their leader Jinbe went and became a privateer for the human-ruled World Government to help patch things up, but the worst case are the New Fishman Pirates. Their leaders hate humans to the extent that they'll kill any fishmen or merfolk who seek peace with humans. Hody Jones, leader of the pirates, is even willing to have all of Fishman Island destroyed than have fishmen and humans understand each other, all out of Irrational Hatred.
In My Immortal, Ebony and friends are obsessed with which characters are "real goffs" and which are just "tryin 2 be goffik". Hagrid, or rather "Hargrid", is reclassified repeatedly, always for nonsensical, arbitrary reasons. How a person becomes a "real goff" never is explained (among a lot of other things), with the closest anyone can guess is that you're born a goff.
In Equestria: A History Revealed, the narrator thinks she's the only legitimate historical essay writer because she's the only one brave enough to tell the "truth", and dismisses all other works of real historians as false. She even calls herself the one true historian in Equestria at one point.
In Dawn of the Planet of the Apes, much like the original movies, there's the Ape Shall Not Kill Ape rule, which Koba uses to taunt Caesar at the end. Caesar responded by declaring that he was no ape. Given how Koba already violated that rule by trying to assassinate Caesar and murdering Ash, this trope holds some real weight for once.
Sansa maintains her belief in a fairytale world of Knights in Shining Armor, despite repeatedly confronting evidence that a knight is really just a thug with a sword, by declaring that any knight who doesn't live up to her expectations and their vows to protect the innocent is obviously "no true knight." She seems to be sticking to this even after acquiring Jade-Colored Glasses by recognizing that true knights, if they exist at all, are very rare. This one is a bit questionable under the "objective guidelines of behavior" exception since, as noted, to become a knight you have to make certain vows to protect the innocent, fight with honor, protect women etc. It might be more accurate to say that "true knights" are actually in the minority in the setting, and indeed, at least one character responds by saying that "there are no true knights."
The Halkans in the Star Trek Novel Verse are total pacifists, who insist that there is no violence of any kind in their hearts. As a result of this, anyone capable of violence cannot be truly Halkan, and will be regarded as a non-person.
Matters of ethnicity are discussed in George MacDonald Fraser's McAuslan trilogy of short stories. Author Avatar Dand McNeill discusses the agonies the Gordon Highlanders went through when accepting a draft of soldiers from the Liverpool Scottish—sons and grandsons of Scotsmen who'd settled in Liverpool but who had the accent of Merseyside rather than Clydeside. One of whom was black, played the bagpipes, and applied to join the Regimental Band. The question was, "can a black-skinned Scouser be said to qualify as Scottish?" closely followed by "What's it going to look like?"
Dand also describes the Pipe-Sergeant in these terms:
If he hadn't been such a decent wee man, he'd undoubtedly have been a professional Scotsman of the most offensive type.
The BFG: The other giants essentially disown the BFG for not eating humans, though neither party does anything to patch the ties. The climax of the film makes it clear that they don't even consider him a giant.
Fleshlumpeater: You is not giant! You is more like...human bean!
The May 5, 2014 segment showed how Republicans running against other Republicans all declare the opponent to be "not a true conservative" by showcasing all the potential definitions thereof and how they are applied in a South Carolina race.
In an episode of Desmond's, Desmond's son is failed on an essay about black British youth, because his teacher thinks it reflects a middle-class background which isn't typical of urban culture. Or as his sister puts it "They're saying he's not black enough!"
Carlton: Wait 'till we come downstairs in these tuxes. People may not think we're twins, but I'll bet they'll think we're brothers. Will: You know, I don't think you'll have to worry about anybody mistaking you for a brother.
Used a bit more seriously in the episode "Blood is Thicker Than Mud", where Will and Carlton try to join an all-black fraternity. Although they're both hazed, Carlton's is more severe than Will's, and even after he endures everything they put him through, the Pledgemaster still refuses to let Carlton join because he thinks he's a "sellout". Will quits in disgust when he finds out, and after they return home and tell Uncle Phil and Aunt Vivian what happened, Phil laments: "When are we going to stop doing this to each other?"
The game Metal Wolf Chaos features propaganda news reports that define a true American as "anyone who supports the idea of having the families and friends of terrorist sympathisers murdered in the streets" rather than "anyone who is a citizen or long-standing resident of America".
In Team Fortress 2, if an Engineer Dominates another Engineer, one possible response is "A real Texan would've dodged that".
Rozalin from Disgaea 2: Cursed Memories loves her father, Overlord Zenon. Until she actually meets him in person, and discovers he's actually kind of a jerk who seems to care for her only as a decorative object to be 'kept safe' in a castle isolated from the outside world. She immediately deduces that he can't be the real Overlord Zenon and is only a fake. She's actually right, but her reasoning that no true Overlord Zenon is a petty jerk had nothing to do with it — the real Overlord Zenon is much worse. Or at least used to be, until reincarnatingas Rozalin.
In The Elder Scrolls V: Skyrim, there's no love lost between the Stormcloaks and their fellow Nords who fight on behalf of the Empire. Apparently, you can't be a "true son/daughter of Skyrim" unless you're one of Ulfric's cronies.
On the other hand, Legate Rikke argues that "Nords have never been fair weather friends" implying that No True Nord would desert their Imperial allies in their hour of greatest need.
In the Borderlands 2 DLC Tiny Tina's Assault on Dragon Keep, Lilith is incredibly reluctant to let Mr. Torgue join in on their game of Bunkers and Badasses because he is a muscular guy, so she believes he's not a true geek and only wants to play because being geeky is "trendy" now.
In Dragon Age: Origins Sten believes in both things and people having an essential nature they are born with and you can't change. For example, if you are born to a merchant clan but become a blacksmith, you are never a blacksmith. You are, forever, a merchant trying to be a blacksmith. If your character is female, Sten comments that you seem to be both a woman and a soldier but this is impossible, you must be either a woman or a soldier.
This gets weird in Dragon Age II DLC, when you meet Tallis, who is both a woman and a warrior and follows the same religion/philosophy that Sten did. This is because the Qun classifies her as a priest rather than a warrior.
In World of Warcraft there are five Horde factions: the playable Horde, which includes all Horde races and not just the orcs like the other examples; the Dark Horde, who are The Remnant of the Warcraft II Horde who invaded Azeroth and are based mostly in Blackrock Mountain and Burning Steppes; the Fel Horde, the still demon-corrupted remnant of the Warcraft II expansion Horde in Outland, mostly alligned with Illidan; and Garrosh Hellscream's True Horde, which is the orc supremacist faction in the Horde civil war during the fourth expansion, all of them which claim to be the rightful Horde excluding the others. The fifth one is the Iron Horde in the fifth expansion, but since that story involves time traveling, their claim of being the rightful Horde was not inaccurate at first.