"Flash in the pan
" is an expression for something which, although initially promising, subsequently proves disappointing.
There are two contenders laying claim to the origins of this phrase:
- Firearms: Flintlock weapons, in common use about two hundred years ago, employed a small primer charge of gunpowder, ignited by sparks from the flint, to cause the main charge of powder in the weapon's barrel to explode. The place where this primer charge was placed was called the "pan", because it was a small, round receptacle with a lid that was replaced after gunpowder was poured in. Use of an insufficient amount of gunpowder would cause a "flash" too small to ignite the main charge and discharge the weapon. "When the trigger was pulled, all he got was a flash in the pan."
- Goldmining: Whilst panning for gold, a miner's eye might catch a glimpse of something sparkling. Is it a nugget of gold? No, it's nothing; it's just a flash in the pan.