"Hoist on his own petard" Meaning?

Question

The meaning is complicated and i once knew it and have fogotten it. Can someone help me?

Answer 1

See #4

pe·tard      /pɪˈtɑrd/   –noun

1. an explosive device formerly used in warfare to blow in a door or gate, form a breach in a wall, etc.

2. a kind of firecracker.

3. (initial capital letter) Also called Flying Dustbin. a British spigot mortar of World War II that fired a 40-pound (18 kg) finned bomb, designed to destroy pillboxes and other concrete obstacles.

—Idiom

4. hoist by or with one's own petard, hurt, ruined, or destroyed by the very device or plot one had intended for another.

[Origin: 1590–1600; < MF, equiv. to pet(er) to break wind (deriv. of pet < L péditum a breaking wind, orig. neut. of ptp. of pédere to break wind) + -ard -ard]