A completely spontaneous urge to shit that has no "holding" potential. You feel it, and you have literally seconds to find somewhere to put it. The actual shit is usually more gaseous and wet than solid doo-doo, and typically explosive. (The term can be used as both a verb and a noun.)
verb: That Thai food was fantastic, but it made me X-Bomb nine times the next day.
noun: I had 12 seconds to find a toilet to release my X-Bomb into, and I couldn't find one. Now I need to change all my clothes.