Origin and history of partner

mid-13c., "division, portion of a whole, element or constituent (of something)," from Old French part "share, portion; character; power, dominion; side, way, path," from Latin partem (nominative pars) "a part, piece, a share, a division; a party or faction; a part of the body; a fraction; a function, office," related to portio "share, portion," from PIE root *pere- (2) "to grant, allot."

It has replaced native deal (n.) in most senses. Meaning "an allotted portion, a share" is from c. 1300; that of "a share of action or influence in activity or affairs, role, duty" is by late 14c. The theatrical sense (late 15c.) is from an actor's "share" in a performance (The Latin plural partis was used in the same sense). In music, "one of the voices or instruments in a concerted piece" (1520s). Sense of "separate piece of a machine" is by 1813.

Meaning "the division of the hair on the head when dressing it; the separation of the hair on the top of the head, from which it spread down on either side" is by 1890, American English; the earlier word for this was parting (1690s). The common Middle English word for it was shede, schede, from Old English scead, scad.

As an adjective from 1590s. Late Old English part "part of speech" did not survive and the modern word is considered a separate borrowing. Phrase for the most part "most, the greatest part" is from late 14c. To take part "participate" is from late 14c.

"accomplice, companion," 1850, a dialectal shortening of pardner, pardener (1795), which represents a common pronunciation of partner (n.).