What is the no true Scotsman fallacy?

Defending a generalization against a counterexample by arbitrarily redefining the group to exclude the exception.

When a counterexample threatens a universal claim, the arguer rescues it by re-drawing the definition so the inconvenient case no longer counts — without any independent justification for the new boundary.

A: "No Scotsman puts sugar on his porridge." B: "My uncle Angus is a Scotsman and he does." A: "Well, no true Scotsman puts sugar on his porridge."

"True" is inserted purely to disqualify the counterexample, making the claim unfalsifiable — there's no real criterion for "true Scotsman" beyond "agrees with my generalization." It's a form of moving the goalposts after the fact.

Tip: Spot it when "real/true/genuine" suddenly appears only to shield a claim from a counterexample — the redefinition does no work except excluding the exception.