be on/off your guard

be off (one's) guard

To not be especially careful or vigilant; to not be prepared for danger or surprises. We'll wait until he's off guard, and then we'll take him and his company for all they're worth! Don't be off your guard, there are a lot of suspicious characters around here!
See also: guard, off

be on/off your ˈguard

be prepared/not be prepared for something, for example an attack, a danger, a surprise, etc. to happen: We must all be on our guard against bomb attacks.He hit me while I was off my guard.
Your guard is a position you take when you want to defend yourself, especially in a sport such as boxing or fencing.
See also: guard, off, on