Vigilant comes from Latin 'vigilans' (stem 'vigilant-'), present participle of 'vigilare' ('to keep awake, watch'), from 'vigil' ('awake, watchful'). In current use it is an adjective meaning "keeping careful watch for possible danger or difficulty". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The -ing ending supplies a participial or action-noun shape. The historical chain includes Latin vigilans, Latin vigilare, and Proto-Indo-European *weǵ-. Related English forms include vigil, vigilante, and invigilate. Derived forms include vigilance and vigilantly. For dictionary purposes, the important point is the continuity between the older form, the route into English, and the present lexical sense.