1. A view stubbornly held in spite of clear evidence that it's wrong.
2. A person who holds such a view.

According to an old story, a priest used the nonsense word mumpsimus (instead of Latin sumpsimus) in the Mass. Even when told it was incorrect, he insisted that he had been saying it for 40 years and wouldn't change it. The expression is "quod in ore sumpsimus" ('which we have taken into the mouth'). Earliest documented use: 1530.

UMPSIMUS - a short-sighted baseball umpire who calls the winning homerun "foul" and then calls the game "over" amidst the ensuing uproar.