"on the watch," 1610s, from French alerte "vigilant" (17c.), from prepositional phrase à l'erte "on the watch," from Italian all'erta "to the height." Second element from erta "lookout, high tower," noun use of fem. of erto, past participle of ergere "raise up," from Latin erigere "raise" (see erect (adj.)).
The adjective is attested from 1712; the noun is from 1796 as "attitude of vigilance" (as in on the alert); 1803 as "a warning report." The verb is from 1868. Related: Alerted; alerting.
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