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" is by 1890, American English; the earlier word for this was parting (1690s).
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.
c. 1200, parten "to depart, leave;" late 13c., "cause (things, persons) to separate;" from Old French partir "to divide, separate" (10c.), from Latin partire/partiri "to share, part, distribute, divide," from pars "a part, piece, a share" (from PIE root *pere- (2) "to grant, allot").
Meaning "divide" (something), especially "divide by cutting or cleaving" is from c. 1300; that of "to share something" (with others) is from early 14c. Of persons, "to separate from one another," early 14c., also intransitive, "draw or hold (persons) apart, separate by intervening." Intransitive sense of "become disunited" is from early 14c.; that of "be divided or severed" is from 1570s. Meaning "to separate the hair, comb the hair away from a dividing line" is attested from 1610s. Related: Parted; parting. To part with "surrender" is from 1580s; earlier it meant "to share with" (mid-13c.).