Green’s Dictionary of Slang

bareback adv.

[the nakedness of the penis; note ride v. (1a)]

[1950s+] having sexual intercourse without using a contraceptive sheath; also occas. as adj.; thus bareback rider, a man who has sex without a condom.

In derivatives

barebacking (n.) (also bareback riding)

[1990s+] hetero- or homosexual intercourse without the use of a condom.