The Holy Hand Grenade of Antioch was one of the sacred relics Brother Maynard of Camelot carried with him on the Quest for the Holy Grail. King Arthur used the Holy Hand Grenade to defeat the rabbit guarding the entrance to the Cave of Caerbannog so that the group could enter.
After a short reading from the Book of Armaments, as transcribed above, King Arthur took the Holy Hand Grenade, pulled the pin, and started counting. He almost ruined the plan when he miscounted: "One, two, FIVE!" Luckily he was quickly corrected. He threw the Holy Hand Grenade at his target, and blew the rabbit to tiny bits. The knights and Brother Maynard were thus able to enter the Cave of Caerbannog to continue their Quest.
Source: Monty Python's excellent movie Monty Python and the Holy Grail.