c. 1300, partie, "a part, division, section, portion," a sense now obsolete; also "physical piece, fragment; section of a book or treatise," 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").
In early use the word often appears where we would have its relative part (n.). Also from c. 1300 in the legal sense "person or group of persons involved in a lawsuit, agreement, etc.," and in the political sense of "a number of persons united in supporting a person, policy, or cause." From early 14c. as any "group of people," also "a social class." Meaning "a person, a paritcular person" is from mid-15c.
The military sense of "a detached part of a larger body or company" is by 1640s. The sense of "a gathering for social pleasure" is found by 1716, from general sense of persons gathered (originally for some specific, temporary purpose, such as dinner party, hunting party).
Phrase the party is over "enjoyment or pleasant times have come to an end" 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 "one who casts gloom over a convivial event" is from 1951, American English.
"have a good time," 1922, from party (n.). Earlier as "to take the side of" (1630s). Related: Partied; partying.
1772, from tea + party (n.). Political references to tea party all trace to the Boston tea party of 1773 (the name seems to date from 1824), in which radicals in Massachusetts colony boarded British ships carrying tea and threw the product into Boston Harbor in protest against the home government's taxation policies. It has been a model for libertarian political actions in the U.S. (generally symbolic), including citizen gatherings begun in early 2009 to protest government spending.
1827, "ignoramus," from know (v.) + nothing. As a U.S. nativist political party, active 1853-56, the name refers to the secret society at the core of the party, about which members were instructed to answer, if asked about it, that they "know nothing." The party eventually merged into the Republican Party. Related: Know-nothingism.
"musical party, private concert or performance," 1872, from French musicale, short for soirée musicale "musical evening (party);" see musical (adj.).