The powder in the idiom refers to gunpowder and not talcum powder — not the `gunpowder’ that we eat with our idlis either! This is the real thing. In the old days, gunpowder and a steel ball had to be pushed down the barrel of a gun, before it could be fired. The powder had to remain dry for the gun to fire. If it got wet, the gun couldn’t be fired. People carrying guns took precautions to keep the powder dry. When you tell someone to keep his powder dry, you are asking him to remain calm and be ready for action. Sir Oliver Cromwell is believed to have coined this expression. Before crossing a river to attack his enemy, he told his troops: “Put your trust in God, but be sure to keep your powder dry.”
*The soldiers were asked to keep their powder dry and await orders. *Don’t panic, you understand? All I am asking you to do is to keep your powder dry.