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.