What is the meaning of argus-eyed?

adjective

• Having very keen vision.
  • usage: "quick-sighted as a cat"
  • synonyms: hawk-eyed, keen-sighted, lynx-eyed, quick-sighted, sharp-eyed, sharp-sighted
• Carefully observant or attentive; on the lookout for possible danger.
  • usage: "a policy of open-eyed awareness"; "the vigilant eye of the town watch"; "there was a watchful dignity in the room"; "a watchful parent with a toddler in tow"
  • synonyms: open-eyed, vigilant, wakeful