off your guard

Not being especially careful or vigilant; not prepared for danger or surprises; being careless or inattentive. He's got a lot of tricky moves, so don't be caught off your guard! We'll wait until he's off his guard, and then we'll take him and his company for all they're worth!

If you are off your guard or off guard, you are not taking actions to protect yourself against trouble. Note: Your guard is the position that you get in when you are ready to defend yourself in sports such as boxing or fencing. Miss Marple is not perceived to be a threat in any way. And, therefore, the criminal is off his guard. He was so pleasant over lunch that I was off guard and spoke freely. Compare with on your guard.