Words related to slap
early 14c., rappe, "a quick, light blow; a resounding stroke," also "a fart" (late 15c.), native or borrowed from a Scandinavian source (compare Danish rap, Swedish rapp "light blow"); either way probably of imitative origin (compare slap, clap).
Slang meaning "a rebuke, the blame, responsibility" is from 1777; specific meaning "criminal indictment" (as in rap sheet, 1960) is from 1903; to beat the rap is from 1927. Meaning "music with improvised words" was in New York City slang by 1979 (see rap (v.2)).