Definition of "listen in" [listen in]

• To listen without participating. (verb)
• To eavesdrop; to listen secretly. (verb)

Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License

• "I used to listen in the deep noon shadow, quiet as the lizards on the pine trunks; sometimes there would be nothing but a wood-dove's coo; but on another day, when the hush was deepest, there would sound far down in the spring a great throat swallowing, or a great mouth smacking its lips together; or sometimes only a long thick breath."
• "During Ekstrom's years working at the Pentagon, he had learned that the Roswell incident had been nothing more than a military accident during a classified operation called Project Mogul – the flight test of a spy balloon being designed to listen in on Russian atomic tests."
• "Though there was never much for her to say, Laila liked to sit and listen in because at these gatherings she was treated to a rare pleasure: She got to hear Mammy speaking affectionately about Babi."