mid-13c., "division, portion of a whole," 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- "to assign, allot" (cf. Greek peprotai "it has been granted," Sanskrit purtam "reward," Hittite parshiya- "fraction, part").
It has replaced native deal (n.) in most senses. Theatrical sense (late 15c.) is from an actor's "share" in a performance (The Latin plural partis was used in the same sense). Meaning "the parting of the hair" is 1890, American English.
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 is from late 14c. To take part "participate" is from late 14c.