Definition -
(1) Stubbornly persisting in using a mispronunciation or a misspelling, etc. after the mistake has been pointed out to you.
(2) A person who does this.

Example -
"I don't care how everyone spells it. I spell it m-i-s-p-e-l-l."

Etymology -
The term was coined in allusion to a once well-known story of an illiterate English priest, who when corrected for incorrectly reading [in the Mass] the phrase quod in ore mumpsimus (the latter a nonsense word) instead of quod in ore sumpsimus (that we have taken up in the mouth), replied, ‘I will not change my old mumpsimus for your new sumpsimus.’

Oxford English Dictionary -
Its first citation is from 1530:
"The chauncelars of englond…which be all lawers and other doctoures mumsimusses of diuinyte were called vpp sodenlye to dispute the mater."
(Tindale Pract. Prelates Hv,)

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