late 13c., "part, portion, side," from Old French partie "side, part; portion, share; separation, division" (12c.), literally "that which is divided," noun use of fem. past participle of 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").
Political sense of "side in a contest or dispute" evolved by 1300; meaning "a person" is from mid-15c. Sense of "gathering for social pleasure" is found by 1716, from general sense of persons gathered together (originally for some specific purpose, such as dinner party, hunting party). Phrase the party is over is from 1937; party line is recorded by 1834 in the sense of "policy adopted by a political party," and by 1893 in the sense of "telephone line shared by two or more subscribers." Party pooper is from 1951, American English.