Fire in the hole

"Fire in the hole" is a warning that an explosive detonation in a confined space is imminent. It originated with miners, who needed to warn their fellows that a charge had been set.

The first cannons developed were discharged, shot or exploded by placing a flaming torch to a small hole packed with gunpowder and leading to the main powder charge. This caused the main charge to explode, propelling the cannon ball to the enemy, or sometimes, blowing up the cannon and all standing nearby.

Hence, fire in the hole was both a command to the torch man, and a warning to all around. Over time cannons improved; they became safer, with no hole or fire needed. The command was reduced to fire, while the full phrase fire in the hole became a general warning for the use of explosive weapons. It was subsequently adopted by the United States Army and Marines to give notice that a grenade or satchel charge was being tossed into a bunker, spider hole, or other enclosure. The term frag out is also commonly used. It is not used for all explosions – throwing a grenade in the open is not announced, for example – only those surprisingly close. It is also commonly used by bomb disposal personnel in both civilian law enforcement and the military to alert nearby persons that a controlled detonation of a suspected bomb or unexploded ordnance is about to take place.

In film and television
This phrase is used extensively on film and TV sets by the special effects department whenever setting off effects charges of any nature (from weapons that fire blanks to a blood squib to huge fireballs). It serves as a warning to the crew that a loud sound is imminent and as a final warning to stop the shot if there are any problems in any department. As an observable example, the phrase is frequently used in MythBusters prior to explosives tests; the explosives technician or another cast/crew member will yell the phrase three times, in three different directions, just before setting off the bomb.

The phrase is also found in many movies. For example:
 * In a deleted scene from Aliens, where Hudson and Vasquez have set up two automated sentry cannons, they use the phrase before throwing out a decoy to test the sentry. The phrase is also used by Hudson in the Aliens movie when a flame thrower dropped by Drake sets light to the inside of the RPC after it has crashed into the Aliens lair.
 * It is used repeatedly by Easy Company soldiers in Band of Brothers episode 2, "Day of Days", when blowing up German 105mm guns during the Brécourt Manor Assault.
 * It is also used by Stella the skunk in the film Over the Hedge before spraying the kitchen while most of the main characters are trapped inside.
 * In the Omaha Beach assault scene of Saving Private Ryan, the phrase is heard right after Bangalore torpedoes are inserted to breach the German defenses.
 * Sarah Connor (Linda Hamilton) uses it to warn her son John before the T-800 blows through a security door of the Cyberdyne Lab in the movie Terminator 2: Judgement Day.
 * It is often used in episodes of the Stargate franchise to warn that explosives are about to be used.
 * It is used when Plankton yelled "FIRE IN THE HOLE!" in "Frozen Face Off" an episode of SpongeBob SquarePants
 * It is used when Skipper yelled "Fire in the hole!" twice, in "Madagascar" and The Penguins of Madagascar.
 * It is also used by Johnny Rico in the movie Starship Troopers when he shot in a "Bug hole" with an atomic rocket.
 * The cast of the television series Sons of Guns occasionally use this prior to test-firing weapons they've built or restored, due to the potential for said weapon to suffer a catastrophic malfunction which may result in an explosion.
 * Jim Breuer's character uses this phrase when he smokes out of a bong in the film Half Baked.
 * JoJo's character says "Fire in the hole!" in RV when trying to clear the blocked sewer drain pipe in the RV.
 * It's also the name of a Chili cookoff in Atlanta Georgia.

In literature
Fire in the Hole is the title of an Elmore Leonard short story, upon which the American FX Channel 2010 television series Justified is partly based.

In video games
This phrase can often be heard in combat-related video games, such as America's Army, Half-Life, Counter-Strike, Far Cry, Battlefield 2, Battlefield 3, Call of Duty, CrossFire, Point Blank, SOCOM II: U.S. Navy SEALs, Left 4 Dead, Left 4 Dead 2, Mass Effect 2, Postal 2, Brothers In Arms, and others.

In OTC products
"Fire in the Hole" is the name of an anti-itch product developed for the US Military and found in drug and grocery stores.