WordWeb Online

1. Having very keen vision
   "argus-eyed as a cat";
   - hawk-eyed, keen-sighted, lynx-eyed, quick-sighted, sharp-eyed, sharp-sighted
    
2. Carefully observant or attentive; on the lookout for possible danger
   "a policy of argus-eyed awareness";
   - open-eyed, vigilant, wakeful

See also: alert, sighted, watchful