off (one's) 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!

Temporarily not careful or vigilant; unprepared for danger or surprises. I'll be honest, the question caught me off guard. I should have been more prepared for the interview. If we catch the defense off guard, this could be a big play.

Also, off one's guard. Not watchful, easily surprised. It is often put as catch (or be caught) off guard, meaning "take (or be taken) by surprise." For example, The securities analyst was caught off guard by that financial report, or With any luck the boss will be off guard when I come in late. [Late 1600s] The antonym, on guard or on one's guard, meaning "watchful or prepared, especially to defend oneself," was first recorded in 1577. For example, In this crowd we must be on guard against pickpockets, or I'm always on my guard when I'm asked how I voted.

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.

Not alert; unprepared.