1. A standard warning indicating that an explosive detonation in a confined space is imminent. (Often used as a military term when a grenade is being thrown into a bunker.)
2. A term used to describe a form of crop dusting when a person farts in an elevator just before getting off so that anyone remaining in the elevator is trapped with the lingering smell until they get to their destination floor.
A fire in the hole can also be used to describe when a person is in an elevator alone and farts just before exiting if the smell lingers long enough to accost the next person who is unfortunate enough to get onto that elevator.
Bob from accounting let a fire in the hole this morning and I couldn't breathe through my nose until we got to the fifteenth floor!