Other forms: eavesdropping, eavesdrops, eavesdropped
To eavesdrop is to listen secretly to what other people are saying. It is always deliberate and always a little shameful — overhearing is accidental, eavesdropping is not, and that distinction is the whole point of having two words.
It takes on: eavesdrop on a conversation. The noun for the person is an eavesdropper. The word is used both literally and about electronic surveillance, where eavesdropping is the standard term for intercepting communications, and it appears in security writing constantly. Compare bug and tap, which are the technical methods, and snoop, which is broader and covers looking as well as listening.
1verbeavesdropTo listen secretly to other people's conversation.
“She was eavesdropping from the landing.”
synonyms: listen in, snoop
2verbinterceptTo intercept private communications.
“The protocol stops anyone eavesdropping on the connection.”
eavesdrop is a way of listening secretly.
The eavesdrop was the strip of ground beside a house where rainwater fell from the eaves. Somebody standing there was close enough to the wall to hear what was said inside — so an eavesdropper was, quite literally, a person found standing in the drip line.
From eaves + drop; the "listening" sense derives from the notion of the listener standing in the area denoted by the physical sense.
Etymology from Wiktionary, CC BY-SA
Practised in Spying & Secrecy, Manners & Etiquette