Why is "eavesdropping" called such?

Answer 1

The "eavesdrop" of a house used to refer to the perimeter of land immediately around it, where rainwater might fall after running off the edge of the roof. "Eavesdropping" originally mean building your house too close to the edge of your property, so that rain ran off onto your neighbor's land and made a mess of it. It later took on the meaning of "trespassing," literally "going within the eavesdrop" of someone else's property without permission. Because the motive of trespassing was so often to listen in on something inside, "eavesdropping" eventually took on its present meaning of "listening in" or "spying."

Answer 2

The term "eavesdropping" originates from when someone was either cleaning or fixing the roof of a building or a house with someone else and those two people are talking to each other, the person on the ground would spy on them by listening to what they are saying by putting their ear close to the end of the gutter pipe, in which the rain travels from the gutters. In this situation, sound waves can ALSO travel from the gutters on top of the roof to the bottom of the gutter pipe near the ground.

Answer 3

Eavesdropping is the intercepting of conversations by unintended recipients. One who participates in eavesdropping (i.e. someone who secretly listens in on the conversations of others) is called an eavesdropper. The origin of the term comes from situations in which people would literally hide out in the eavesdrop of a house to listen in on private conversations.
History
Eavesdropping was already prohibited by ancient Anglo-Saxon law. From the Saxon custom arose the term eavesdropping, as being one who pries into the business of others or listens to private conversations. By common law, an eavesdropper was regarded as a nuisance. The offense was punishable by fine. Though the offense of eavesdropping still exists in common law, there is no modern instance of a prosecution or indictment.