apartment (n.) Look up apartment at Dictionary.com
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.