No true Scotsman

This is a logical fallacy I see referred to off and on in Great debates. What is it exactly?

From the context I gather it has something to do with the illegitimate exclusion of contradictory examples as in:

A: All Scotsmen drink.

B: Angus McFeely has never touched a drop in his life.

A: Then he is not a true Scotsman.

But I’m not sure about this. Could I get some help please?

Link

Another link

Thanks, ascenray

Yer welcome.