to listen to the radio
to intercept radio communications
to listen but not contribute (to a discussion), esp surreptitiously
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Other Idioms and Phrases with listen in
Hear or overhear the conversation of others; eavesdrop. It is also put as listen in on, as in She listened in on her parents and learned they were planning a surprise party. [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.