not guarded or protected

You shouldn't leave your bag unguarded like that. 你不能就那样把你的包放着没人管。

If you make an unguarded remark or say something in an unguarded moment, you tell someone something that you would usually keep secret, sometimes in a way that shows bad judgment.

In an unguarded moment, I said that I didn't trust Charles. 一不留神，我脱口而出说我不信任查尔斯。