In military terms, an absolute dud is a nuclear weapon which fails to detonate or does not explode when it lands on a target. Despite the fact that the weapon appears to be nonfunctional, it can still be extremely dangerous, requiring the attention of people who are trained in the disposal of unexploded ordinance. More generally, the term “dud” is used to describe any type of failed weapon, and civilians have picked up the term as well, describing an ignominious failure as an "absolute dud." The implication is often that the failure is also expensive, as nuclear weapons take a long time to develop and build, and processing unexploded weapons properly is also costly.
The origins of the word “dud” have been traced back to the 1300s, when the word was first used to describe a cloak. The word evolved to mean “ragged or dirty” clothing, and by the 1800s, it was used to refer to things that were useless and sometimes expensive as well. “Dud” in reference to weapons emerged in the First World War, when soldiers often struggled with shells with failed to explode, or exploded too early.
There are a number of reasons for a nuclear weapon to turn out to be an absolute dud. Facilities which manufacture such weapons take numerous steps to avoid creating duds, but sometimes weapons simply malfunction with no clear explanation. Modern nuclear weapons are extremely complex, with numerous places in their circuitry and wiring where a failure could originate. Since nuclear weapons are used primarily in testing, rather than in active warfare, an absolute dud is more irritating and potentially dangerous than anything else, but in wartime, the failure of such a weapon to detonate could be devastating; the enemy, for example, might be able to extract nuclear material from the weapon and use it.
Any sort of dud is potentially dangerous because such a weapon could still explode. Duds need to be handled carefully, as the cause for their failure to detonate is unknown, and they may be highly unstable. Most militaries have specialized bomb disposal units which deal with duds and other explosives, ensuring that they are safe before allowing people to handle them. Generally, the less professionally something is made, the more likely it will be a dud; amateur explosives fans can be at risk of serious injuries from duds, for example.
Generally, a military will not publicize an absolute dud. The failure of a weapon to launch or detonate is not very good for morale, and it can suggest that a nation's weapons manufacturing facilities may not be the best, potentially illuminating a weakness which could be exploited by an opponent.