To fail or end in a disappointing way, often quietly or gradually, or to make a weak hissing or sputtering sound.
From Middle English 'fise' (to break wind) combined with the suffix '-le' (to make a repeated sound). The word originally described an embarrassing bodily sound, but gradually came to mean any weak, disappointing ending—the onomatopoeia shifted from crude to comic to metaphorical.
The phrase 'fizzle out' became popular during the fireworks era—it describes firecrackers that don't explode properly but just sputter out with a sad hiss. This is why 'fizzle' now means any disappointing anticlimax, whether it's a party, a career, or a romantic relationship.
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