A fly on the wall意思

"A fly on the wall" is an English idiom that means to be a hidden observer of something private or confidential. It suggests the idea of being unseen and unnoticed, much like a fly on a wall would be, and thus able to hear or see everything that is happening without being a part of it or affecting it in any way.

This expression is often used to describe someone who is eavesdropping or spying on a conversation or event, or it can refer to a situation where someone has managed to gain insider knowledge or a unique perspective on something by being in the right place at the right time.

For example, if you say that you were "a fly on the wall" during a meeting, it means you were there observing everything that was said and done without participating or being noticed.