What is the X in X-mas?
late 14c., "carried away in an ecstatic trance," from Latin raptus, past participle of rapere "seize, carry off" (see rape (v.)). A figurative sense, the notion is of "carried up into Heaven (bodily or in a dream)," as in a saint's vision. Latin literal sense of "carried away" was in English from 1550s. In 15c.-17c. the word also sometimes could mean "raped." Sense of "engrossed" first recorded c.1500. As a past participle adjective, in English it spawned the back-formed verb rap "to affect with rapture," which was common c.1600-1750.
c.1300, "a quick, light blow, stroke," also "a fart" (late 15c.), native or borrowed from a Scandinavian source (cf. Danish rap, Swedish rapp "light blow"); either way probably of imitative origin (cf. slap, clap).
Slang meaning "rebuke, blame, responsibility" is from 1777; specific meaning "criminal indictment" (cf. rap sheet, 1960) is from 1903. To beat the rap is from 1927. Meaning "music with improvised words" first in New York City slang, 1979 (see rap (v.2)).
mid-14c., "strike, smite, knock," from rap (n.). Related: Rapped; rapping. To rap (someone's) knuckles "give light punishment" is from 1749. Related: Rapped; rapping.
"talk informally, chat," 1929, popularized c.1965 in Black English, possibly first in Caribbean English and from British slang meaning "say, utter" (1879), originally "to utter a sudden oath" (1540s), ultimately from rap (n.). As a noun in this sense from 1898. Meaning "to perform rap music" is recorded by 1979. Related: Rapped; rapping.
A form of pop music characterized by spoken or chanted rhymed lyrics, with a syncopated, repetitive accompaniment. Rap music originated in the second half of the twentieth century in black urban communities. (See also hip-hop.)
[origin unknown; perhaps related to repartee, perhaps to rapport, perhaps to rapid]
["An Interpreter for a Language for Describing Assemblies", R.J. Popplestone et al, Artif Intell 14:79-107 (1980)].