Oh. Thanks. I must be thinking of a different word.
Petard was a small explosive bomb used to breach a wall or door or gate in early modern warfare.
However, when petard is used as an idiom, it means to be undone by (or be the victim of) one's own schemes.
Hoisted by or on one's own petard means caught by the very device one had contrived to hurt another.
The idiomatic usage originated with Shakespeare's play, Hamlet, written in 1600.
Hamlet, upon learning of a plot to kill him, contemplates how to turn the tables against the plotters, Rosencrantz and Guildenstern.
The phrase "hoist with one's own petard" thus meant "to be blown up with one's own bomb." It became a commonplace expression in 17th century English.