put one on guard
put (one) on guard
To cause one to assume a position, attitude, or mindset of wary caution. A possessive pronoun can be used between "on" and "guard." A slower-than-expected financial quarter has put some investors on guard. The unnerving silence of the small town put me on my guard.
put one on one's guard
to make one wary or cautious. The menacing growl by the watchdog put me on my guard. Every time the boss comes around, it puts me on my guard.