While there is more than one defination, I believe that the common usage is that of a common animal trap, consisting of a rope attached to a bent over tree or branch that trips when an animal steps into it and it whips into the air, ensnaring the creature in the process.
Set it off by accident?
Hoist by your own petard!
Thanks to both of you. Freepers know everything.
Very close. A petard is actually an explosive device. If you set it off by accident, it will indeed "hoist" you!