mid-13c., counseiler, "one who gives counsel or advice, a confidante," from Old French conseillier "counselor, adviser" (Modern French conseiller), from Latin consilator, agent noun from consiliare, from consilium (see counsel (v.)).
Also sometimes counsellor, but the double -l- is unetymological and perhaps is modeled on chancellor. Meaning "one who gives professional legal advice, a counseling lawyer," is from 1530s. Psychological sense (as in marriage counselor, is from 1940).
