The word eavesdropper existed before the word eavesdrop. Yep, our current word for "to listen in on others" is actually a seventeenth-century back-formation from a previous term meaning "one who listens at walls or windows". This in turn was created from the noun eavesdrip, which is the "place where water drips off the roof". Thus, an eavesdropper is someone who hides near an eavesdrip and listens to conversations going on inside. The first part of that, eaves, is a word that still exists today, meaning "lower part of a roof". Eaves, through Old English efes and Proto-Germanic ubaswo, derives from the Proto-Indo-European reconstruction upo, or "under". Drip, obviously, we also have in our vernacular. It traces to Proto-Indo-European dhreu through Proto-Germanic drupjanan, both with the same meaning. The word eavesdrop has been increasing exponentially in recent years, as people have been getting nosier, I guess.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.