Etymology
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.

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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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au pair (n.)
1897 of the arrangement, 1960 of the girl; French, literally "on an equal footing" (see au + pair (n.)).
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Bethlehem 
the name probably means "House of Lahmu and Lahamu," a pair of Mesopotamian agricultural deities.
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clasp-hook (n.)

"pair of hooks provided with a ring which can hold them together," 1841, from clasp (n.) + hook (n.).

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covalent (adj.)

1927, from covalence "the linking of two atoms by a shared pair of electrons" (1919), from co- + valence.

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tabla (n.)
pair of drums used in northern Indian music, 1865, from Hindi, from Arabic tabl "a drum played with the hand."
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claves (n.)

"pair of hardwood sticks used in making music," 1928, from American Spanish claves (plural), from Spanish clave "keystone," from Latin clavis "key" (from PIE root *klau- "hook").

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yoke (n.)
Old English geoc "contrivance for fastening a pair of draft animals," earlier geoht "pair of draft animals" (especially oxen), from Proto-Germanic *yukam (source also of Old Saxon juk, Old Norse ok, Danish aag, Middle Dutch joc, Dutch juk, Old High German joh, German joch, Gothic juk "yoke"), from PIE root *yeug- "to join." Figurative sense of "heavy burden, oppression, servitude" was in Old English.
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doubly (adv.)

"in a double or twofold manner, in two different ways, as a pair," c. 1400, from double (adj.) + -ly (2).

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