The generally accepted story of the word's origin is found in the 1517 writing of Richard Pace, a humanist and friend of Sir Thomas More. Pace later became the Dean of St Paul's Cathedral in London.
Pace tells of a medieval monk who persisted in saying "quod in ore mumpsimus" instead of "quod in ore sumpsimus" when celebrating mass. “Sumpsimus” is Latin for "we have taken," and the full phrase translates to "which we have taken into the mouth.” “Mumpsimus” is just babble.
It isn't clear whether the well-seasoned monk was illiterate (though that is the general assumption) or whether the word was transcribed incorrectly in his copy of the mass. What made this particular mistake memorable is that when a younger monk tried to correct the old guy, the older man replied that he had been saying it that way for over forty years and added, "I will not change my old mumpsimus for your new sumpsimus."
And that is how ‘mumpsimus’ came to mean:
A. a person who persists in a mistaken expression or practice.
B. an erroneous practice, use of language, or belief that is obstinately adhered to.
Not sure if you wanted to know, but now you do.
Contributed by: Dean Lindsay, Award Winning Speaker and Author of The Progress Challenge & Co-Author of Stepping Stones to Success