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1.
(noun) Term used among marijuana smokers describing a marijuana seed that was accidentally rolled into the joint or blunt or loaded into the pipe. When the cherry reaches the hippy bomb and heats it to a certain temperature, it "explodes" making a sharp snapping or popping sound that isn't very loud at all but is often quite startling while under the influence of pot, especially to the one smoking it at the time.
*POP*
Stoner: Whoa, gotta watch out for those hippy bombs!
by Sean Quixote December 04, 2005
 
2.
A hippybomb is usually referring to a pot seed that has been (or will be smoked) smoked, usually resulting in a large headache/migraine.

A pot seed that has been rolled into a Joint or packed into a Bowl/pipe, on accident, resulting in a migraine.
Hey man, don't pack that Hippy Bomb into the bong.
by Got Purple February 24, 2010