1640s, "private rooms for the use of one person or family within a house," from French appartement (16c.), from Italian appartimento, literally "a separated place," from appartere "to separate," from a "to" (see ad-) + parte "side, place," from Latin partem (nominative pars) "a part, piece, a division" (from PIE root *pere- (2) "to grant, allot").
Sense of "set of private rooms rented for independent living in a building entirely of these" (the U.S. equivalent of British flat) is by 1863, with reference to Paris. Apartment house is attested from 1870.
*perə-, Proto-Indo-European root meaning "to grant, allot" (and reciprocally, "to get in return"); possibly related to *pere- (1) "to produce, procure."
It forms all or part of: apart; apartment; bipartient; bipartisan; bipartite; compartment; depart; department; ex parte; impart; jeopardy; multipartite; parcel; parse; part; partial; participate; participation; particle; particular; particulate; partisan; partition; partitive; partner; party; portion; proportion; quadripartite; repartee; tripartite.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit purtam "reward;" Hittite parshiya- "fraction, part;" Greek peprotai "it has been granted;" Latin partem (nominative pars) "a part, piece," portio "share, portion."
men's apartment in a house, from Greek andron, collateral form of andronitis "men's apartment," from anēr (genitive andros) "man" (from PIE root *ner- (2) "man"). The female equivalent was a gynaeceum.
1817, "composed of two parts, double, twofold," from Latin duplex "twofold," from duo "two" (from PIE root *dwo- "two") + -plex, from PIE root *plek- "to plait." The noun in the sense of "house or other building so divided that it forms two dwelling places" (also sometimes "two-story apartment") is American English, by 1922.
1640s, from French concierge "caretaker, doorkeeper of a hotel, apartment house, prison, etc., porter" (12c.), of uncertain origin. Perhaps from Vulgar Latin *conservius, from Latin conservus "fellow slave," from assimilated form of com "with, together" (see con-) + servius "slave" (see serve (v.)). In France, formerly the title of a high royal official. Related: Conciergerie (c. 1600).
1801, "a story of a house," from Scottish flat "floor or story of a house," from Old English flett "a dwelling, hall; floor, ground," from Proto-Germanic *flatja-, from suffixed form of PIE root *plat- "to spread." Meaning "floor or part of a floor set up as an apartment" is from 1824. Directly from flat (adj.) come the senses "level ground near water" (late 13c.); "a flat surface, the flat part of anything" (1374), and "low shoe" (1834).