1. Something that does not work as intended, is ineffectual, or is defective. This car I bought yesterday is a complete dud. It's engine won't even start!
2. Someone who is disappointing or unsuccessful. He turned out to be a bit of a dud. He doesn't have any plans to get a job; he just wants to watch TV all day.
dud
(dəd)
n. a failure; something that fails to perform as intended. (see also duds.) The whole idea turned out to be a dud.