a stubborn person who insists on making an error in spite of being shown that it is wrong
About the word:
Supposedly, this insult originated with an illiterate priest who said mumpsimus rather than sumpsimus ("we have taken" in Latin) during mass. When he was corrected, the priest replied that he would not change his old mumpsimus for his critic’s new sumpsimus.
Part of M-W’s Top Ten Rare and Amusing Insults