In Nethack, YAAD is an abbreviation for Yet Another Annoying Death. It is different from YASD (Yet Another Stupid Death) in that YASD typically refers to a death that could have been avoided with more care or knowledge of Nethack, such as choking to death on a wraith corpse or being killed by a newt while paralysed by a floating eye, while YAAD typically refers to death from unavoidable bad luck. A typical YAAD would be when everything is going fine until...

You fall into a pit!
You land on a set of sharp iron spikes!
The spikes were poisoned!
The poison was deadly....
Do you want your possessions identified?