Dictionary.net

a traditional notion that is obstinately held although it is unreasonable; "he still holds to the old mumpsimous that a woman's place is in the kitchen"

mump'si-mus, n. an error to which one clings after it has been thoroughly exposed. [Corr. of L. sumpsimus, in the mass, by an ignorant priest who refused to correct it.]

It is said of some priest that lie insisted on reading mumpsimus for the L. sumpsimus, we have received, in the prayer after Communion. Hence the word came to denote the obstinacy of ignorance.