pair (n.)

mid-13c., paire, "a set of two, 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., "a couple, a sexual pair." Used from late 14c. with a plural noun to denote a single tool or device composed essentially of two pieces or parts (shears, tongs, spectacles, etc.). Meaning "a woman's breasts" is attested from 1922. Pair bond (v.) is first attested 1940, in reference to birds mating.

pair (v.)

"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 "separate from a company in pairs or couples" (1783) for the former and pair up (1863) for the latter. Related: Paired; pairing.

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Definitions of pair from WordNet
1
pair (v.)
form a pair or pairs;
The two old friends paired off
Synonyms: pair off / partner off / couple
pair (v.)
bring two objects, ideas, or people together;
The student was paired with a partner for collaboration on the project
Synonyms: match / mate / couple / twin
pair (v.)
occur in pairs;
Synonyms: geminate
pair (v.)
arrange in pairs;
Synonyms: geminate
pair (v.)
engage in sexual intercourse;
Synonyms: copulate / mate / couple
2
pair (n.)
a set of two similar things considered as a unit;
Synonyms: brace
pair (n.)
two items of the same kind;
Synonyms: couple / twosome / twain / brace / span / yoke / couplet / distich / duo / duet / dyad / duad
pair (n.)
two people considered as a unit;
pair (n.)
a poker hand with 2 cards of the same value;
From wordnet.princeton.edu