mumpsimus
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.
sumpsimuses
1. Adherence to or persistence in using a strictly correct term, holding to a precise practice, etc., as a rejection of an erroneous but more common form.
2. A person who is obstinate or zealous about such strict correctness.