(To Be a) Fly on the Wall

What does the idiom “(To Be a) Fly on the Wall” mean?

Definition:

To be an unnoticed observer

For example:

“I would have liked to be a fly on the wall when Judy confronted her boyfriend about cheating.”

Notes:

Inasmuch as it’s usually impossible to be unobserved in a room, this is usually expressed as a wish, as in the example.

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