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!
be on/off your ˈguardbe 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.