Fire in the hole

"Fire in the hole" is a standard warning, used in many countries in the world, indicating 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 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 term “fire in the hole” became a general warning for the use of explosive weapons.

The phrase 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. It is not used for all explosions — throwing a grenade in the open is not announced, for example — only those surprisingly close.

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 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 a result the phrase is frequently used in "MythBusters" due to their extensive work on set as Special Effects technicians.

"Fire in the Hole" is also a name for a drive-through prank in which a someone orders a cup of soda at a drive-through window, and then opens it and throws it back through the window at the restaurant employee. Restaurant employees and management were understandably annoyed at this behavior, and many now request a ticket-ready police presence during late-night and graveyard shifts.

It's sometimes sarcastically used to indicate when a fart is imminent, as a warning of the waste gases to be given off imminently.