Alert (&adot_;l&etilde_;rt), a. [F. alerte, earlier à l'erte on the watch, fr. It. all' erta on the watch, prop. (standing) on a height, where one can look around; erta a declivity, steep, erto steep, p. p. of ergere, erigere, to erect, raise, L. erigere. See Erect.] 1. Watchful; vigilant; active in vigilance.
[1913 Webster]

2. Brisk; nimble; moving with celerity.
[1913 Webster]

An alert young fellow. Addison.
[1913 Webster]

Syn. -- Active; agile; lively; quick; prompt.
[1913 Webster]