A booby-trap is a hidden device designed to surprise or harm someone who triggers it accidentally. The word combines “booby” (an old term for a foolish person) with “trap,” suggesting a trap that catches people who aren't paying attention.

In wartime, soldiers sometimes set booby-traps like hidden explosives that detonate when someone opens a door or steps on a concealed trigger. These dangerous devices are meant to slow down or injure enemies who enter an area.

In everyday life, people sometimes use the term more playfully. For example, in stories or movies, a character might set a harmless booby-trap by balancing a pillow above a door so it falls when someone walks in, or by rigging a bucket of water to tip over when a door opens. These pranks are called booby-traps because they catch the victim by surprise.

The key feature of any booby-trap is that it activates automatically when someone disturbs it, without anyone needing to be there to spring it. That's what makes it different from simply hiding and jumping out to scare someone: the trap does the work itself. Even playful booby-traps can upset people or cause accidents.

As a verb, to booby-trap something means to set a hidden device so it will go off when someone touches, opens, or moves it.