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1.
A cheat code in the iD Software game Doom, which gives you all the weapons in the game, as well as all the keys.
Type idkfa to stock up on those precious BFG cells!
by ChaoX December 17, 2004
 
2.
A well-known cheat code for the computer games Doom and Doom 2. Entering the code gives the player all keys, all weapons, and full ammunition. It can be entered an unlimited number of times per level to refill ammo.

The cheat code most likely means:

id = id Software
k = keys
fa = full ammo
I can't believe they left the classic Doom cheat codes like idkfa and iddqd out of Doom 3!
by MC Mikey July 02, 2011
 
3.
Cheat code for Doom.

Acronym stands for "I Don't Know Fuck All"
Man, where's all the loot at, IDKFA?
by Buttersx July 16, 2011