A phrase still much used, it usually follows along with the meaning of 'Take Cover' -- something big and bad is about to happen, so watch out and keep your legs and arms inside the ride at all times.
The research I've done seems to point to the origin being military in nature, like so many others. In par with "cold enough to freeze the balls off a brass monkey", it has to do with cannon.
Older cannons, prior to the 20th century, were simply metal tubes with two holes in them; one at the business end, which was used for loading and unloading, and the breach hole (sometimes called the 'nipple') which was used for igniting the propellant that lay inside, behind the cannonball.
In the early days of munitions, to light the propellant, usually black powder, a flaming stick was unceremoniously jammed in the breach hole, igniting the powder, thereby moving the ordnance in such a manner as to cause a severe reaction to all those in the vector of said cannonball.
Of course, in the intrest of saving your own troops' lives, you would give a warning to those in earshot to not only get out of the way of the shot, but to move far enough away from the cannon at to not be run over by it as it moved backward from the tremendous recoil of the weapon.
Later usage was adopted other aspects of war, namely the dropping of hand grenades down enemy foxholes or trenches. The 'fire' (grenade) was in the 'hole' (the enemy barracks).
Isn't this a pathetic
writeup to move up
to level 7 with?