eavesdrop on (someone or something)
To intentionally listen to a conversation that one is not a participant in. Because it's likely that people will eavesdrop on our discussion, I think we should postpone it until a later time. If you don't stop eavesdropping on us, I'm going to tell mom!
eavesdrop on someone
to listen in on people having a private conversation. I saw her eavesdropping on them. Please don't eavesdrop on me.
eavesdrop on something
to listen in on a private conversation. She was eavesdropping on their conversation. Maria was eavesdropping on the telephone call.