1814, diminutive from sermon + -ette. Poe used sermonoid (1849); sermuncle (1886) also has been tried. English writers have turned to the Italian double diminutive sermonettino (1818) to describe notably trifling efforts.
c. 1200, sarmun, "a discourse upon a text of scripture; what is preached," from Anglo-French sermun, Old French sermon "speech, words, discourse; church sermon, homily" (10c.), from Latin sermonem (nominative sermo) "continued speech, conversation; common talk, rumor; learned talk, discourse; manner of speaking, literary style," originally "a stringing together of words," from PIE *ser-mo-, suffixed form of root *ser- (2) "to line up."
Main modern sense in English and French is elliptical for Latin sermo religiosus. In transferred (non-religious) use from 1590s. The Sermon on the Mount is in Matthew v-vii and Luke vi. Related: Sermonic; sermonical; sermonish.
diminutive word-forming element, from Old French -ette (fem.), used indiscriminately in Old French with masculine form -et (see -et). As a general rule, older words borrowed from French have -et in English, while ones taken in since 17c. have -ette. In use with native words since late 19c., especially among persons who coin new product names, who tend to give it a sense of "imitation, a sort of" (for example flannelette "imitation flannel of cotton," 1876; leatherette, 1855; linenette, 1894). Also in such words as lecturette (1867), sermonette, which, OED remarks, "can scarcely be said to be in good use, though often met with in newspapers."
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Harper, D. (n.d.). Etymology of sermonette. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/sermonette