Fire in the hole is a warning that an explosion is about to occur. The phrase fire in the hole originated in the American coal mines where miners used dynamite to loosen the rock. Dynamite was detonated in small spaces, and miners used the warning fire in the hole to alert their fellow workers to take cover. In fact, mining regulations call for the warning fire in the hole to be verbalized three times before ignition. Today, the military has adopted the term to warn of an impending explosion of ordnance, and NASA uses fire in the hole when talking about igniting multi-stage rockets. Fire in the hole is sometimes used in a slang fashion, to mean watch out.
Fire in the hold is a misunderstanding of the phrase fire in the hole.
Outdoors, Hugh Brown, ordnance sergeant for the 36th Texas Cavalry of Saginaw, cried, “Fire in the hole!” and triggered a .58-caliber musket loaded with 65 grains of black powder. (The Temple Daily Telegram)
A few moments later, with Walthour safely returned to a bomb squad van, a team member shouted, “Fire in the hole!” (The Star Press)
No sense in waiting during wartime, Retseck said. “Fire in the hole!” he yelled. (The Times Free Press)
City workers and onlookers counted down to the demolition, with crews crying out “fire in the hole” before the explosives were detonated. (The Telegraph)
Dutch gets to yell, “Fire in the hole!” as she drops a silver-filled grenade onto an elevator shaft full of resting vampires, and Fet uses his handy rebar to stab a strigoi hiding in a crawl space. (The Los Angeles Times)
A neighbor heard authorities yelling, “Fire in the hole! Fire in the hole!” followed by a loud noise. (The Roanoke Times)