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.