A Flash in the Pan

A literal mechanical failure from the age of flintlock firearms. A flintlock musket ignited its main charge by first sparking a small amount of gunpowder held in a metal "pan" beside the barrel; that spark was meant to travel through a small touch-hole and set off the main charge, firing the shot. Sometimes the priming powder in the pan would flare up in a bright flash without ever igniting the main charge, producing a dramatic burst of light, smoke, and noise but no actual shot fired, all promise and no result. The phrase is recorded in this literal sense from the 1680s, with figurative use for disappointing false starts following by the early 1800s.