preach (v.)

1540s, "a word," a sense now obsolete, from Late Latin dictionem (nominative dictio) "a saying, expression; a word; kind of delivery, style," noun of action from past-participle stem of Latin dicere "to say, state, proclaim, make known, allege, declare positively" (source of French dire "to say"), which is related to dicare "to talk, speak, utter, make speech; pronounce, articulate," both from PIE root *deik- "to show," also "pronounce solemnly." The meaning "manner of saying," especially in reference to the choice of words, is from 1700.

Latin dicere and dicare are presumed to have been originally the same word. De Vann writes that "the verb dicāre may well have been backformed from compounds in -dicāre." The basic sense in both is "to talk, speak, declare." They seem to have divided, imperfectly, the secondary senses between them: dicere "to say, state, proclaim, make known, allege, declare positively; plead (a case);" in religion, "to dedicate, consecrate," hence, transferred from the religion sense, "give up, set apart, appropriate;" dicare "to talk, speak, utter, make speech; pronounce, articulate; to mean, intend; describe; to call, to name; appoint, set apart."

"one authorized or appointed to discourse publicly on religious subjects," c. 1200, prechour, from Old French preecheor "preacher" (Modern French prêcheur), from Latin praedicatorem (nominative praedicator) "public praiser, eulogist," literally "proclaimer" (see preach). Slang short form preach (n.) is recorded by 1968, American English.