eavesdropper (n.)youdaoicibaDictYouDict[eavesdropper 词源字典]
mid-15c., with agent-noun ending + 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."[eavesdropper etymology, eavesdropper origin, 英语词源]