late 15c., "play or sing in harmony," from French harmoniser (15c.), from Old French harmonie (see harmony). Meaning "be in harmony (with), go well together" is from 1620s. Transitive sense "bring into harmony" is from 1700; figurative sense "bring into agreement" is from 1767. Meaning "add harmony to (a melody)" is from 1790. Related: Harmonized; harmonizing.