One who listens stealthily to conversation. The derivation of the term is not usually understood. The owners of private estates in Saxon times were not allowed to cultivate to the extremity of their possessions, but were obliged to leave a space for eaves. This space was called the yfes-drype (eaves-drip). An eaves-dropper is one who places himself in the eaves-drip to overhear what is said in the adjacent house or field.
Under our tents I'll play the eaves-dropper, To hear if any mean to shrink from me.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894