I love to look at phrases and interoperate them in the form of a picture. Looking around my bungalow I found these two items, a pan and a light in a bottle, this gave me my Flash in a Pan.
So what's it's meaning - Well it's something which disappoints by failing to deliver anything of value, despite a showy beginning. Many of us ladies have found this in past with ex-boyfriends or partners. I also expect our 365 gentlemen can come up with the same comment, I wouldn't want to get in an argument about this.
So where does this phrase originate from, well I like the story that comes from the Californian Gold Rush. A prospector was panning for gold, when suddenly he spotted a glint in his pan, gold he thought, but then couldn't find anything, hence the phrase, "It was just a Flash in the Pan".