The BFG 9000 is a famous gun in the Doom series, and the most powerful gun in the game. The BFG shoots a ball of plasma that detonates on contact with a solid object, and once it goes off, additional bursts of plasma damage every single enemy currently within eyesight of the player who fired it. Many games have had guns inspired by the BFG 9000, and Quake II even featured a more advanced model, the BFG 10,000.
In the Doom Bible - the game's original design and concept document, written by Tom Hall - a weapon referred to as "BFG 2704 (red)" is described as "Horrible hallway-scouring weapon. Damages user a bit. Awful recoil. BFG stands for "Big Fucking Gun." Safest use: back to wall, distant target."
The BFG 9000 is a one shot kill for all enemies, even in ultra-violence mode, except Cyberdemons (three shots) and Spider Masterminds (two shots).
The BFG 9000 returns in Doom 3, where it is once again the most powerful weapon in the game. Unlike its original appearance, where it shared Energy Cell ammunition with the Plasma Rifle, the Doom 3 version uses its own unique ammunition, simply referred to as "BFG Cell", which is of course as rare as hen's teeth, given the developers' unfortunate decision to make Doom 3 a survival horror game. This version of the BFG 9000 can also be charged for even greater damage and a larger blast radius, but if charged for too long, the gun will explode and kill the player.
In the movie Doom, the BFG 3.14 is a weapon discovered by Sarge, which operates similarly to the classic gun aside from shooting blue projectiles instead of green ones, and leaving behind acidic residue on surfaces it's touched. It's mentioned that the acronym here stands for "Bio-Force Gun". In a nod to the most popular interpretation of the BFG acronym, after firing the weapon out of curiosity, Sarge exclaims, in surprise, "Big... fucking... gun." Tragically, the effects of the weapon on living creatures are never actually shown.
Hank Leukart 's "Official" Doom FAQ (1994) claims the F to stand for Fraggin': "Being the most powerful weapon, the only thing BFG9000 could stand for would be "Big Fraggin' Gun." :) "