Vigilant.

synoniemen: on guard, on your guard, upon one's guard.

Roget 864: cautious, wary, guarded; on one's guard etc. (watchful) 459; cavendo tutus [Lat.]; in medio tutissimus [Lat.]; vigilant.    careful, heedful; cautelous^†, stealthy, chary, shy of, circumspect, prudent, discreet, politic; sure-footed etc. (skillful) 698.    unenterprising, unadventurous, cool, steady, self-possessed; overcautious.    ... meer laten zien

Roget 459: careful regardful, heedful; taking care etc. v.; particular; prudent etc. (cautious) 864; considerate; thoughtful etc. (deliberative) 451; provident etc. (prepared) 673; alert etc. (active) 682; sure-footed.    guarded, on one's guard; on the qui vivre [Fr.], on the alert, on watch, on the lookout; awake, broad awake, vigilant; watchful, wakeful, wistful; Argus-eyed; wide awake etc. (intelligent) 498; on the watch for (expectant) 507.    tidy etc. (orderly) 58, (clean) 652; accurate etc. (exact) 494; scrupulous etc. (conscientious) 939; cavendo tutus [Lat.] etc. (safe) 664.    ... meer laten zien

Roget 668: warning etc. v.; premonitory, monitory, cautionary; admonitory, admonitive^†; sematic [Biol.].    warned, forewarned etc. v.; on one's guard etc. (careful) 459, (cautious) 864.    ... meer laten zien