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vigilant (adj.)

late 15c., from Middle French vigilant or directly from Latin vigilantem (nominative vigilans) "watchful, anxious, careful," present participle of vigilare "to watch, keep awake, not to sleep, be watchful," from vigil "watchful, awake" (from PIE root *weg- "to be strong, be lively"). Related: Vigilantly.

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Definitions of vigilant from WordNet

vigilant (adj.)
carefully observant or attentive; on the lookout for possible danger;
the vigilant eye of the town watch
Synonyms: argus-eyed / open-eyed / wakeful
From wordnet.princeton.edu