Diehard of a sort

Wikipedia Word definitions in Wikipedia
A mumpsimus is an action by a person who adheres to a routine, idea , custom , set of beliefs, or a certain use of language that has been shown to be unreasonable or incorrect. For example, a person may continue to say all intents and purposes as all intensive ...

WordNet Word definitions in WordNet
n. 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"

Wiktionary Word definitions in Wiktionary
n. 1 A person who obstinately adheres to old ways in spite of clear evidence that they are wrong; an ignorant and bigoted opponent of reform. 2 An obvious error that is obstinately repeated despite correction.