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1.
A grenade, often in FPS games, that does not go where intended that may:

1) Bounce back and harm and/or kill you.

2) Miss the target completely.
I heaved a disgruntled sigh as I was obliterated by my failnade.
by HTMLHTML January 17, 2009
 
2.
In the Counter-Strike Source Mod known as Zombie Mod, an improperly thrown grenade (or any grenade thrown by a n00b) results in failnade, which instead of hurting the zombies, propels them forward into whatever spot you are trying to defend, resulting in failure.
The BirdMan, YeaH!: Fire in the hole!

ZOMBIEMAN X--> The Birdman, YeaH!
ZOMBIEMAN X--> chucknorris101
ZOMBIEMAN X--> ewbernoob3000

chucknorris101: Nice failnade Birdman...
The BirdMan, YeaH!: =D
by The BirdMan, YeaH! July 15, 2008