How is “listen in” used?

In Fargo (1996), one line is: “You know, they said no one listening in.”

Which movie is “listen in” from?

The clip at the top of this page is from Fargo (1996).

You know, they said no one listening in.

8 作品で登場 · 1 本目

to listen to a conversation that you are not supposed to hear

9 本の映画に登場

オックスフォード映画の 100%
to listen to a conversation that you are not supposed to hear
Well, like any decent, self-respecting housekeeper, I listen in on the other phone. I just don't know what to say.
- Shut the door. - Don't forget, those dames listen in.

定義：Oxford Advanced Learner's Dictionary 第 9 版