before 900; (noun) Middle Englishevesdrope,evesdripe,Old Englishyfesdrype; as v., probably back formation from eavesdropper,late Middle Englishevisdroppyr, apparently literally, one who stands on the eavesdrop in order to listen to conversations inside the house; see eave, drop, drip
mid-15c., from Middle English eavesdrop, from Old English yfesdrype "place around a house where the rainwater drips off the roof," from eave (q.v.) + drip (v.). Technically, "one who stands at walls or windows to overhear what's going on inside."
c.1600, probably a back-formation from eavesdropper. Related: Eavesdropping.