Secretly listening to other people's conversations without their knowledge or permission.
From 'eavesdrop,' possibly from 'eaves' (the edge of a roof) and 'drop' (to fall). In medieval times, people would stand under the eaves of a house to secretly listen to conversations inside. First recorded in English in the 1600s.
The word 'eavesdropper' originally literally meant someone standing in the dripping zone under roof gutters, which is such a vivid image—it shows medieval people were already thinking about privacy and boundaries in buildings.
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