2. Something which disappoints by failing to deliver anything of value, despite a showy beginning
This originally had a literal meaning, i.e. a real flash in a real pan. Muskets used to have small pans to hold the gunpowder charge. An attempt to fire the musket in which the gunpowder flared up without firing a bullet would be called a 'flash in the pan'.
origin, The term is known since the late 17th century. Elkanah Settle, in Reflections on several of Mr. Dryden's plays 1687, had this to say:
"If Cannons were so well bred in his Metaphor as only to flash in the Pan, I dare lay an even wager that Mr. Dryden durst venture to Sea."