Def, you mean this?
Big Lie (disambiguation).
A big lie (German: große Lüge) is a propaganda technique and logical fallacy. The expression was coined by Adolf Hitler, when he dictated his 1925 book Mein Kampf, about the use of a lie so “colossal” that no one would believe that someone “could have the impudence to distort the truth so infamously”.
https://en.wikipedia.org/wiki/Big_lie
:: a lie so colossal that no one would believe that someone could have the impudence to distort the truth so infamously. ::
Replaced, today, by the term “conspiracy theory”.