Mumpsimus: a word I just discovered that I had never read or heard before. It has an interesting etymology.
Frequency (in current use):
Origin: A borrowing from Latin. Etymon: Latin mumpsimus.
Etymology: < post-classical Latin mumpsimus (1517 in R. Pace De Fructu), use as noun of mumpsimus , an error for classical Latin sumpsimus ‘we have taken’ (see sumpsimus n.), apparently in allusion to the story (1516 in Erasmus) of an illiterate English priest, who when corrected for reading ‘quod ore mumpsimus’ in the Mass, replied, ‘I will not change my old mumpsimus for your new sumpsimus’.(Show Less)
a. A person who obstinately adheres to old ways in spite of clear evidence that they are wrong; an ignorant and bigoted opponent of reform.
*Bold emphasis added