Its definition sort of indeterminate, even within the game. The game (Doom) was developed by id Software, which I assume lends the "id" prefix. Most places cite "Keys and Full Ammo" for the rest, or "Killer Fucking Ammo" in some places.

Really, though, it was an available, game-related, nerdy domain name that me and my father came up with back on August 20th, 1999.

Huh. Which is today. Happy birthday, idkfa.