Instead of "quod in ore sumpsimus", he would say "quod in ore mumpsimus". Now 'sumpsimus' is Latin for "we have taken" (the full phrase means "which we have taken into the mouth"), but 'mumpsimus' is just nonsense.

What made this particular mistake memorable is what the monk was supposed to have said when he was corrected. According to the version of the incident told in 1517 by Richard Pace, later the Dean of St Paul's Cathedral in London, the monk replied that he had said it that way for forty years and "I will not change my old 'mumpsimus' for your new 'sumpsimus'".

As a result, the word came to be applied to someone who sticks obstinately to their old ways, in spite of the clearest evidence that they are wrong. The word can also have the related meaning of some custom or notion that is adhered to, even though it has been shown to be unreasonable.

Some references suggest that the story may have been first told by Pace's friend Erasmus; there's also a hint that it may really have been an oft-told joke in medieval times. The word is first recorded in 1530 in a book by William Tyndale, the first translator of the Bible into English, called _The Practice of Prelates_. It was given royal approval in 1545, when Henry VIII referred to it in a speech:

"Some be too stiff in their old mumpsimus, others be too busy and curious in their sumpsimus".

It has, perhaps surprisingly, stayed in the language as a learned joke for more than 400 years, and is still to be stumbled over from time to time.