One who derives pleasure, usually sexual, from secretly spying on others. (Seevoyeurism.)
The original “peeping Tom” was a legendary resident of the town where Lady Godiva rode naked through the streets. According to the story, Tom defied official orders by looking out his window as she went by and was struck blind.
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.