listen quietly, without contributing to the conversationlisten without the speaker's knowledge
listen in Idiom, Proverb
1．listen to a radio broadcast，etc．听广播等 We were listening in to the President's speech．我们在收听总统的广播讲话。 He likes to listen in late at night．他喜欢在深夜听广播。 2．listen to the talk of others 偷听（讲话）；窃听（电话） When Mary talked to her boy friend on the telephone，her little brother listened in．玛丽与男朋友打电话时，她弟弟在偷听。 The police had tapped his tele phone wire and listened in on his conversations with his confederates．警方在他的电话线上搭线，监听他和同伙的谈话。 The boy listened in while we were discussing the problem．我们在讨论这个问题时，这男孩在旁边听着。
v. 1. To listen to a radio broadcast. We found them listening in to the president's speech. 2. To listen to the talk of others, often to talk that is not intended for your ears; eavesdrop. When Mary talked to her boyfriend on the telephone, her little brother listened in.