Answer: mumpsimus

AnswerCrossword Clue
MUMPSIMUSDiehard of a sort
mumpsimusadherence to or persistence in an erroneous use of language, memorization, practice, belief, etc., out of habit or obstinacy.
mumpsimususing an erroneous use of language, out of habit or obstinacy.
mumpsimusan erroneous word or spelling that, through long usage, has become absorbed into the language; an incorrect manuscript reading blindly adhered to by some scholar