(n) a traditional notion that is obstinately held although it is unreasonable, Example:he still holds to the old mumpsimus that a woman's place is in the kitchen
The Collaborative International Dictionary of English (GCIDE) v.0.53 Collaborative International Dictionary (GCIDE)
n. [ Prov. E. mump to be sulky. Cf. Mump, Mumble, and Mum. ] 1.pl. Sullenness; silent displeasure; the sulks. Skinner. [ 1913 Webster ]
2. [ Prob. so called from the patient's appearance. ] (Med.) A specific infectious febrile disorder characterized by a nonsuppurative inflammation of the parotid glands, and sometimes causing inflammation of the testes or ovaries; also called epidemic parotitis or infectious parotitis. It is caused by infection with a paramyxovirus. [ 1913 Webster +PJC ]