A grandson asked about the origin of this expression, because he apparently has heard it often on the Military Channel. The Word Detective appeals that the phase not be shouted as a silly prank and the serious history of the expression begs that it only be used when appropriate. Underground coal miners have used the expression as a warning of a planned detonation since early in the twentieth century, and mining laws of several states require the warning. Military bomb disposal teams adopted the expression in the 1940s, and it continues to be used by the military to warn that a detonation is about to occur.