1. Something that appears to be harmless but which is set up so that it will explode and kill anyone who touches it: The apartment had booby traps that were rigged to go off if anyone touched a certain light switch, used a handle to open one or more of the doors to rooms, etc.

2. A humorous contraption that is set up to shock or to scare someone: Ken made a booby trap for his guests by making the bathroom door lock when anyone closed it and it could not be unlocked by the person who would go in to wash his or her hands, or to use any of the other facilities.

To put what appears to be a harmless object somewhere so it can explode and cause harm to anyone who touches it: The shooter at the theater in Aurora, Colorado, on July 20, 2012, had booby trapped his apartment and it took at least two days before the police could neutralize the explosives so they could safely enter the place to gather evidence.

• James Holmes, the Aurora, Colorado shooter, who reportedly opened fire at a Batman movie premiere, admitted to police that he had booby trapped his apartment with explosives.
• When police searched Holmes apartment, they discovered that it had been booby trapped and so they evacuated not only the building where his apartment was, but also the surrounding buildings.