Mumpsimus: a person who obstinately adheres to old ways, customs, or ideas despite clear evidence they’re wrong or outdated. Also refers to the error or practice itself.
Etymology: From a 16th-century story about an English priest who mispronounced “sumpsimus” (Latin: “we have received”) as “mumpsimus” in Mass for decades. When corrected, he refused to change, saying “I will not change my old mumpsimus for your new sumpsimus.”
The word captures stubborn adherence to error even after correction—distinct from simply being wrong or uninformed.