listen in

US /ˈlɪs.ən ɪn/
UK /ˈlɪs.ən ɪn/
"listen in" picture
1.

to listen to a private conversation between other people, without them knowing that you are listening

:
•
I caught my sister trying to listen in on my phone call.
•
The neighbors often listen in on our conversations.
:
eavesdropoverhearspy onmonitortap
:
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