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)).
mid-14c., rappen, "to strike, smite, knock," from rap (n.). Related: Rapped; rapping. To rap (someone's) knuckles "give sharp punishment" is from 1749 (to rap (someone's) fingers in the same sense is by 1670s.). Related: Rapped; rapping.
intransitive, "talk informally, chat in an easy way," 1929, according to OED, popularized c. 1965 in African-American vernacular, possibly first in Caribbean English and from British slang rap (v.) "to say, utter" (by 1879), originally "to utter sharply, speak out" (1540s), ultimately a sense-branch of rap (v.1).
As a noun in this sense from 1898. Meaning "to perform rap music" is recorded by 1979. Related: Rapped; rapping.