noun

• (informal) A device or machine that is useless because it does not work properly or has failed to work, such as a bomb, or explosive projectile.
  "The only amusing highlight was Gudgeon having managed to exploit U.S. codebreaking efforts to ambush and destroy the submarine I-173, albeit not for the lack of the Mark 14's trying to sabotage the effort, as the torpedo that had hit the sub had refused to detonate; it seemed, however, that the car-crash levels of kinetic energy involved in the dud simply ramming the sub had nonetheless done enough to fatally damage it."
• (informal) A failure of any kind.
  "At the end of the day, the vast majority of primary schools are vibrant, friendly places and you may struggle to choose one because they all seem so great. Primary schools tend to have the feelgood factor. If you just aren't feeling it, this one's probably a dud."
• (informal) A failure of any kind.
  ""Now you can't fool me. Tom, I'm not one o' those duds that pay you a thick 'un for an hour in the dark.""
• (informal) A failure of any kind.