Etymology From eaves + drop; the "listening" sense derives from the notion of the listener standing in the area denoted by the physical sense.
1
Ambitransitive To hear (intentionally) a conversation one is not intended to hear; to listen in.
You shouldn't eavesdrop, David. I told you: You'll only hear things you'll wish you hadn't.1999 February 3, Anthony Horowitz & al., "Strangler's Wood", Midsomer Murders
I hope nobody was eavesdropping on our conversation last night, since it got so personal.
2
Biology, Natural sciences, Zoology To listen for another organism's calls, so as to exploit them.
[…] Eastern Red Bat will eavesdrop on the sonar calls of other red bats to locate potential insect prey.1993, David W. Nagorsen, Robert Mark Brigham, Royal British Columbia Museum, Bats of British Columbia, page 109:
Frog-biting midges also depend on eavesdropping for reproduction, and one would predict similarly strong selection to evolve auditory adaptations for hearing and localizing calls of their hosts.2014, Ken Yasukawa, Animal Behavior: How and Why Animals Do the Things They Do: