1705 as abbreviation of reputation (n.); upon rep "I swear it" was a common 18c. slang asseveration. As a shortening of repetition (n.) it is recorded from 1864, originally school slang; as a shortening of representative (n.), especially "sales representative," it is attested from 1896. As an abbreviation of repertory (company) it is recorded from 1925.
Roentgen-equivalent-physical; a unit of absorbed radiation dose, equal to the amount of ionizing radiation that will transfer 93 ergs of energy to 1 gram of water or living tissue.
Reputation: gettin' the rep a not havin' a big schnozz (1705+)
A representative: The sales rep from Kokomo (1896+)
: Both of whom are repped by yours truly
: a rep company
Repertory: She played in rep a couple years (1925+ Theater)
A repetition: I mused on this while I did 15 reps at 250 (1864+)