be on/off your guard

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