sermonize v.

c. 1200, sermoun, sarmun, "a discourse upon a text of scripture; that which is preached," from Anglo-French sermoun, Old French sermon, sermun "speech, words, discourse; church sermon, homily" (10c.) and directly from Latin sermonem (nominative sermo) "continued speech, conversation; common talk, rumor; learned talk, discourse; manner of speaking, literary style."

This is reconstructed to be from PIE *ser-mo-, a suffixed form of the root *ser- (2) "to line up," hence "to thread, thread together," and thus "a stringing together of words." De Vaan writes, "The derivation from a root 'to link, put on a string' is not compelling, but can be defended with parallel etymologies of words for 'speech' in other languages."

The sense in English and French is elliptical for Latin sermo religiosus "public religious discourse." Throughout Middle English the word also was used in its non-religious classical sense of "a discourse, a discussion." In modern transferred (non-religious) use, it is from 1590s. Also by late 13c. of particular discourses of Christ, the Apostles, etc. Related: Sermonic; sermonical; sermonish. Sermoner "preacher" is attested from early 13c. as a surname.