Wiktionary
listen in
vb. 1 To listen without participating. 2 (context idiomatic English) To eavesdrop; to listen secretly.
WordNet
listen in
v. listen quietly, without contributing to the conversation
listen without the speaker's knowledge; "the jealous man was eavesdropping on his wife's conversations" [syn: eavesdrop]
Usage examples of "listen in".
All communication between Hyperborea and Good Intentions had been routinely coded, as well as tight-beamed, on the assumption that berserkers or goodlife could be almost anywhere, and anything that could make it harder for them to listen in was worth a try.
You were sent here by your brother to ask me to listen in on people's thoughts when they're in the bar, so I can find out about this orgy thing Lafayette went to.