"to banish, send out of one's native country," 1768, modeled on French expatrier "banish" (14c.), from ex- "out of" (see ex-) + patrie "native land," from Latin patria "one's native country," from pater (genitive patris) "father" (see father (n.); also compare patriot). Related: Expatriated; expatriating. The noun is by 1818, "one who has been banished;" main modern sense of "one who chooses to live abroad" is by 1902.
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Definitions of expat from WordNet
a person who is voluntarily absent from home or country;