Mumpsimus

A mumpsimus () is a "traditional custom obstinately adhered to however unreasonable it may be", or "someone who obstinately clings to an error, bad habit or prejudice, even after the foible has been exposed and the person humiliated; also, any error, bad habit, or prejudice clung to in this fashion". Thus it may describe behaviour or the person who behaves thus. For example, "all intensive purposes" is a common eggcorn of the fixed expression "all intents and purposes"; if a person continues to say the eggcorn even after being made aware of the correct form, either the speaker or the phrase may be called a mumpsimus.
Origin.
The term originates from an apocryphal story about a poorly educated Catholic priest saying Latin mass who, in reciting the postcommunion prayer (meaning: "What we have received in the mouth, Lord"), instead of (meaning: "we have received") substitutes the non-word "mumpsimus", presumably as a mondegreen. After being made aware of his mistake, he nevertheless persisted with his erroneous version, whether from stubbornness, force of habit, or refusing to believe he was mistaken.
The story was told by Desiderius Erasmus (1466–1536) in a letter he wrote in August 1516 to Henry Bullock. Erasmus used it as an analogy with those who refused to accept that "Novum Instrumentum omne", his edition of the Greek New Testament, corrected errors in the Latin Vulgate. The English diplomat Richard Pace (1482–1536) included a variant in his 1517 work "De Fructu qui ex Doctrina Percipitur", where the priest was English and had been saying "mumpsimus" for thirty years when corrected. While Pace's book (written in Latin) is credited by the first edition of the "Oxford English Dictionary" as the origin of "mumpsimus", Pace acknowledged his borrowing in a 1517 letter to Erasmus. "Mumpsimus and sumpsimus" became proverbial among Protestants in the early English Reformation.
Usage.
Mumpsimus soon entered the language as a cant word widely used by 16th-century writers.
In William Tyndale's 1530 book "Practice of Prelates", the word was used in the sense of a stubborn opponent to Tyndale's views. He said that the men whom Cardinal Wolsey had asked to find reasons why Catherine of Aragon was not truly the wife of King Henry VIII of England were "all lawyers, and other doctors, mumpsimuses of divinity". Sir Thomas Elyot in 1531 in "The Book of the Governor" explains why he uses the term "good courage" instead of "magnanimity" thus: "this worde Magnanimitie beinge yet straunge, as late borowed out of the latyne, shall nat content all men, and specially them whome nothing contenteth out of their accustomed Mumpsimus, I will aduenture to put for Magnanimitie a worde more familiar, callynge it good courage".
Henry VIII in his speech at the State Opening of Parliament on Christmas Eve 1545 said:
Peter Heylin refers to the king's saying in his 1631 "The History of St. George of Cappadocia" when he talks of "those self-conceited ones which are so stiffe—as King Harry used to say—in their new sumpsimus..." 
Hugh Latimer (1487–1555) used the term in two sermons he preached in 1552, saying: "When my neighbour is taught, and knoweth the truth, and will not believe it, but will abide in his old "mumpsimus"..." and again: "Some be so obstinate in their old "mumpsimus", that they cannot abide the true doctrine of God."
In an 1883 polemic on errors in translations of the Christian Bible, John Burgon says: "If men prefer "their" 'mumpsimus' to "our" 'sumpsimus', let them by all means have it: but pray let them keep their rubbish to themselves—and at least leave our SAVIOUR's words alone."
Eugene T. Maleska, 1970s editor of "The New York Times" crossword puzzle, received "dozens of letters" after "mumpsimus" appeared as an answer; he had felt that "it was time to revive the obsolete noun". A. Leslie Derbyshire applied it in a 1981 management science book to managers who know how to do a better job but choose not to. "Garner's Modern English Usage" says the word could describe George W. Bush because of his persistent habit of pronouncing "nuclear" as "nucular", despite the error being widely reported.