c. 1300, "in a state of readiness" (an adjectival sense, now obsolete), literally "fully ready, quite prepared," a contraction of all ready; see all + ready (adj.). Compare Norwegian, Danish allerede "already." As an adverb, "by this time, previous to some specified time," late 14c. The colloquial use in U.S. as a terminal emphatic (as in enough, already!) is attested from 1903, translating Yiddish shoyn, which is used in same sense. The pattern also is attested in Pennsylvania German and in South African.
"feeling of having previously experienced a present situation," 1903, from French déjà vu, literally "already seen." The phenomenon also is known as promnesia. Similar phenomena are déjà entendu "already heard" (of music, etc.), 1965; and déjà lu "already read" (1960).