intransitive verb
1. : to tune in to or monitor a transmission
listened in last night to Continental stations — B.L.K.Henderson
listen in on the submarine-bell receivers for the noises made by the propellers of passing vessels — Scientific American
listen in on the enemy's communications line
2. : to give ear to a conversation without participating in it
I remember listening in while a foreman and his girl workers were discussing who was going to work — Sam Pollock
especially : eavesdrop
listen in on a party line
when they speak from a telephone booth in a hotel, everybody sitting in the lobby listens in — A.T.Weaver