rap (n.)

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)).

rap (v.1)

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.

rap (v.2)

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.

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Definitions of rap from WordNet
rap (n.)
a reproach for some lapse or misdeed;
it was a bum rap
Synonyms: blame
rap (n.)
a gentle blow;
Synonyms: strike / tap
rap (n.)
the sound made by a gentle blow;
Synonyms: pat / tap
rap (n.)
voluble conversation;
rap (n.)
genre of African-American music of the 1980s and 1990s in which rhyming lyrics are chanted to a musical accompaniment; several forms of rap have emerged;
Synonyms: rap music / hip-hop
rap (n.)
the act of hitting vigorously;
Synonyms: knock / belt / whack / whang
rap (v.)
strike sharply;
rap him on the knuckles
Synonyms: knap
rap (v.)
make light, repeated taps on a surface;
Synonyms: tap / knock / pink
rap (v.)
perform rap music;
rap (v.)
talk volubly;