c.1600, from Fr. alerte "vigilant," from phrase à l'erte "on the watch," from It. all'erta "to the height," from erta "lookout, high tower," orig. fem. of erto, pp. of ergere "raise up," from L. erigere "raise." The adv. is the oldest form in Eng.; the adj. is from 1712, the noun from 1803, and the verb from 1868.