Английский Этимологический словарь - rap (v.)

- "talk informally," first recorded 1929, popularized c.1965 in Black English, possibly first in Caribbean English, from British slang meaning "say, utter" (1879), originally "to utter a sudden oath" (1541), from rap (n.). Meaning "music with improvised words" first in New York City slang, 1979.