peephole

Definitions

A hole or crevice in a wall through which a person can watch another, usually a woman, undress or engage in sexual activities.

ETYMOLOGY: The term dates back to 1681.

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(Noun) A hole or crevice in a wall/door through which one person can see another while being unobserved oneself. (No sexual connotation is necessarily implied. The word dates back to at least 1675, but is such natural/inevitable usage that an earlier origin is easily possible.)