Википедия поясняет, когда используется эта фраза:
Using grenades
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When a grenade is out of control, yell "grenade." When a grenade is dropped into a enclosed space like a tunnel, the dropper should yell "
Fire in the hole!" to alert his comrades that an explosive is about to detonate.
Или Dictionary of Marine Corps Terms and Words Used in Vietnam:
FIRE IN THE HOLE!: warning that explosives are about to be detonated.
Что касается происхождения выражения, на experts.about.com :
Question — Where doed the expression "fire in the hole" come from?
Answer — the origin is debated, but most like came from blasting in mining operations, and made it's way to military use as well. It is used in other things, but most famous for those applications.
А в Pirate Glossary утверждается, что
Fire in the Hole — The warning issued before the gunner set his match to the
powder hole on a cannon to
fire it.