"hoist with his own petard"

     [This is a fast-access FAQ excerpt.]
"For 'tis the sport to have the enginer / Hoist with his owne
petar" -- Shakespeare, Hamlet III iv.  "Hoist" was in Shakespeare's
time the past participles of a verb "to hoise", which meant what "to
hoist" does now:  to lift.  A petard (see under "peter out" for the
etymology) was an explosive charge detonated by a slowly burning
fuse.  If the petard went off prematurely, then the sapper (military
engineer; Shakespeare's "enginer") who planted it would be hurled
into the air by the explosion.  (Compare "up" in "to blow up".)  A
modern rendition might be:  "It's fun to see the engineer blown up
with his own bomb."