on guard

on (one's) guard

Especially careful or vigilant; prepared for danger or surprises. He's got a lot of tricky moves, so be on your guard! Those trails can be a bit treacherous to climb. I just hope she's on her guard.
See also: guard, on
Farlex Dictionary of Idioms.

*on (one's) guard (against someone or something)

alert against someone or something. (*Typically: be ~; keep ~; remain ~; stay ~.) Try to stay on guard against pickpockets. I am always on my guard. Be on guard when you go into the city.
See also: guard, on
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

on (one's) guard

Alert and watchful; cautious.
See also: guard, on
American Heritage® Dictionary of the English Language, Fifth Edition.
See also:
  • on (one's) guard
  • on one's guard
  • on your guard
  • always be on (one's) guard
  • off (one's) guard
  • off guard
  • off your guard
  • catch (one) off guard
  • catch off guard
  • catch one off