No true Scotsman is a term coined by Antony Flew in his 1975 book Thinking About Thinking. It refers to an argument which takes this form:
Argument: "No Scotsman puts sugar on his porridge."
Reply: "But my uncle Angus likes sugar with his porridge."
Rebuttal: "Ah yes, but no true Scotsman puts sugar on his porridge."
This form of argument is a fallacy if the predicate ("putting sugar on porridge") is not actually contradictory for the accepted definition of the subject ("Scotsman"), or if the definition of the subject is silently adjusted after the fact to make the rebuttal work.
This is the exact defense the Rebbe used at about the same time after Lubavitch thugs harassed and threatened Rabbi Rifkin, the lone member of the then-newly-formed Chabad Va'ad HaRabbanim who refused to put an Israeli politician from the National Religious Party into cherem for failing to heed the Rebbe's (unasked for) instructions. The thugs broke Rabbi Rifkin's windows, made threatening and harassing phone calls in the middle of the night, spit on him while walking the streets of Crown Heights, and made this very elderly man's life a living hell.
Appeals to the Rebbe to call off his dogs went unheeded. After weeks of unabated harassment, Rabbi Rifkin's son-in-law wrote about the harassment in the Yiddish Forward and provided pictures of the swastikas painted on Rabbi Rifkin's house. The Yiddish press (with the exception of Chabad's mouthpiece, the Algemiener Journal) went wild and demanded answers from 770.
Argument: The Rebbe told the reporters that no Chabad chassid, no one who learns Chabad chasidus, would act that way.
Reply: The reporters pointed out the clear Chabad connection, including the Rebbe's long angry rants at public gatherings about the "one who would not sign" the excommunication order, and that the Rebbe's staff had posted the order with the missing signature all around 770 before those rants, so no one would be confused about which rabbi to harass.
Rebuttal: The Rebbe repeated his claim, adding to it that no true chasid would act that way.
This is a defense haredi leaders use today. No "truly frum yid" would cover up for a pedophile or defraud a bank or investors or cheat the government, etc., etc. But the facts are that fraud and other crimes are rampant in the haredi world. Today it may be more reasonable to claim "no truly frum yid" would file an honest tax return or refrain from committing welfare fraud than claiming the opposite.
And the fault of that rests solely on the rabbis and community leaders who have shown them the way.