c.1500, "to make a loud, sharp sound," of imitative origin, or a blend of clap and crash. Cf. Dutch kletsen "splash, clash," German klatschen, Danish klaske "clash, knock about." Figurative sense, in reference to non-physical strife or battle, is first attested 1620s. Of things, "to come into collision," from 1650s; of colors, "to go badly together," first recorded 1894. Related: Clashed; clashing.
1510s, "sharp, loud noise of collision," from clash (v.). Especially of the noise of conflicting metal weapons. Meaning "hostile encounter" is from 1640s; meaning "conflict of opinions, etc." is from 1781.