Dud (noun)

1. A thing that fails to work or function properly.
2. A bomb or missile that fails to explode.
3. A person or thing that is worthless or ineffective.
4. A failure or disappointment.

Origin:

Early 19th century (in the sense "spurious coin"): of uncertain origin.

Examples:

1. The fireworks display was a dud, most of the rockets failed to ignite.
2. The new product was a dud and failed to sell well.
3. He was a dud of a teacher, unable to hold the attention of his students.
4. I was disappointed when my new phone turned out to be a dud and wouldn't turn on.
5. He felt like a dud, unable to make any progress with his project.