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.
reorganization
reorganize
reorient
reorientation
reovirus
rep
repaint
repair
repairable
reparable
reparation