No true Scotsman (appeal to purity) is an informal fallacy in which one attempts to protect their universal generalisation from a falsifying counterexample by excluding the counterexample improperly. Rather than abandoning the falsified universal generalisation or providing evidence that would disqualify the falsifying counterexample, a slightly modified generalisation is constructed ad-hoc to definitionally exclude the undesirable specific case and counterexamples like it by appeal to rhetoric - emotionally charged but nonsubstantive purity platitudes - like "true, pure, genuine, authentic, real" etc. In short: an "ad hoc rescue" of a refuted generalization attempt.
Simplified rendition of the fallacy:
Appeal to purity is used here to protect/elevate a preferred group. "No true Scotsman would do XYZ" or "Only in Scotland would there be XYZ".