"to come together with another; be mated or married" (intransitive), also "to make a pair by matching" (transitive), c.1600, from pair, n.. These senses now often are distinguished by pair off (c.1803) for the former and pair up (1908) for the latter. Related: Paired; pairing.

mid-13c., "two of a kind coupled in use," from Old French paire "pair, couple," and directly from Medieval Latin paria "equals," neuter plural of Latin par (genitive paris) "a pair, counterpart, equal," noun use of par (adj.) "equal, equal-sized, well-matched" (see par, n.). Originally of things. Of persons from late 14c. Meaning "a woman's breasts" is attested from 1922. Pair bond (v.) is first attested 1940, in reference to birds mating.