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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- (3) "to line up, join" (see series).
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 5,6,7 Matt. and 6 Luke. Related: Sermonic; sermonical; sermonish.