mumpsimus \MUHMP-suh-muhs, noun:

1. Adherence to or persistence in an erroneous use of language, memorization, practice, belief, etc., out of habit or obstinacy.
2. A person who persists in a mistaken expression or practice.

“I profess, my good lady,” replied I, “that had any one but you made such a declaration, I should have thought it as capricious as that of the clergyman, who, without vindicating his false reading, preferred, from habit’s sake, his old Mumpsimus…
— Sir Walter Scott, The Talisman

Mr. Burgess, who sticks (I fancy) to his old mumpsimus, thought that the other gentleman might have given the canoe a shove to get it clear of the lock…
— Ronald A. Knox, The Footsteps at the Lock

Mumpsimus comes from a story (perhaps first told by Erasmus) about an illiterate priest who mispronounced a word while reciting the liturgy. The priest refused to change the word, even when he was corrected.