mumpsimus
 
mump·si·mus
muhmp-suh-muhs
noun, plural mump·si·mus·es for 2.
1.
adherence to or persistence in an erroneous use of language, memorization, practice, belief, etc., out of habit or obstinacy (opposed to sumpsimus ).
2.
a person who persists in a mistaken expression or practice (opposed to sumpsimus ).

Origin:
1520–30; from a story, which perhaps originated with Erasmus, of an illiterate priest who said mumpsimus rather than sūmpsimus (1st plural perfect indicative of Latin sūmere to pick up; see consume) while reciting the liturgy, and refused to change the word when corrected

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