peeping Tom

A man who secretly observes women undressing or engaging in sexual intercourse. Unless you want to be a target for peeping Toms, you better get curtains for your bedroom windows soon.

A person who secretly watches others, especially for sexual gratification; a voyeur. For example, The police caught a peeping Tom right outside their house. This expression, first recorded in 1796, alludes to the legend of the tailor Tom, the only person to watch the naked Lady Godiva as she rode by and who was struck blind for this sin.