A dud is an ammunition round or explosive that fails to fire or detonate, respectively, on time or on command.
Duds are still dangerous and have to be deactivated and disposed of carefully. Poorly designed devices (for example, improvised explosive devices (IEDs), and small devices, have higher chances of being duds. "Device Unable to Detonate" could be a possible acronym, or at least a backronym, for dud.
The variation absolute dud describes a nuclear weapon that fails to explode.[1]
Although dud rounds have failed to function as designed, they are still dangerous, and can explode if handled. In wartorn areas, many curious children have been injured or killed from tampering with such devices. Dud fireworks should be destroyed by soaking in water.
By extension, "dud" has become a slang word for anything that does not work or is defective. There is also a candy called Milk Duds, so named because it was impossible to get them perfectly round.
Generally your duds are "your last possessions", what you are wearing and carrying, so nice duds is ironic and possibly an oxymoron. Other meanings of 'dud' are: