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mump·si·mus (mŭmpsə-məs)
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n. pl. mump·si·mus·es
1. One who adheres stubbornly to a mistaken practice or usage despite having been corrected by others.
2. A practice or usage that is adhered to in this way.

[Originally a mistaken pronunciation of Latin sūmpsimus, we have taken (first person plural perfect indicative of sūmere, to take; see SUMPTUARY), occurring in a popular story (first known from a letter written by Desiderius Erasmus in 1516) in which an ignorant parish priest, when corrected after having said mumpsimus instead of sumpsimus in the post-communion prayer of the Mass for twenty years, stubbornly refuses to change his habits and use the correct pronunciation.]

Usage Note: One might be tempted to form the plural of mumpsimus as mumpsimi, as if it were a Latin second-declension noun like alumnus, focus, or nucleus. But mumpsimus is not a Latin word of any sort; it is merely a mangling of the real Latin word sumpsimus, and sumpsimus is not a noun either but a first-person plural verb. The more proper plural for mumpsimus in English, therefore, is the regularly formed mumpsimuses.

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.
 

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.