Where did the phrase "drop the hammer" come from?

I've only ever seen it as described in definition-of.com: "Bringing a pending act to fruition. Usually connotes an act which will have serious consequences" and always as a one-time act which will bring all force to bear.

Also, like loufedalis' answer given in Yahoo! Answers, I believe the origin to be military and related to the hammer of a cocked pistol or rifle. When you pull the trigger and drop the hammer that certainly fits the poetic description of someone waiting for the right moment to strike with all available force.