misfire; failure to fire
Used for guns, explosives, or engines that fail to ignite or fire. Also used metaphorically for plans or efforts that fail to materialize.
The gun misfired.
The plan fizzled out.
The firework failed to go off, and the audience sighed.
空振り literally means a swing-and-miss and is used for an effort that produces no effect, while 不発 focuses on something failing to ignite or launch, whether literal or metaphorical.
不振 describes an ongoing slump or poor performance (e.g. a batter in a slump), whereas 不発 refers to a single failure to fire or produce an effect.
unexploded ordnance; UXO; unexploded bomb/shell
Compound of 不 (fu, 'not') and 発 (hatsu, 'discharge' or 'fire'). The reading ふはつ follows standard on'yomi.