Stands for The Big Friendly Giant and is a common nickname for your dick.
Travis: So dude, you gonna be putting the moves on Ashley tonight?
Mike: Yeah but all I'll have to do is whip out The B.F.G. and he'll take it from there.
by B-more Bro June 5, 2007
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Big Fucking Gun. A POWERFUL munitions device seen mainly in Hollywood action and science-fiction movies, first sighted in possession of Colonial Marines in James Cameron's epic "Aliens" (1986); now seen in almost every dark-themed science-fiction or action film since. Dark, hulking, made of questionable metal alloys that do not exist yet. Fires: plasma, explosives, Big Fucking Bullets, anything it fucking well feels like.

Also seen in various anime films.

A popular user option in "Doom", and used vigorously in the feature film version of the game. Other names: Standard-Issue Big Gun, S.I.B.G., Big-Ass Gun, B.A.G., copwaster. For more details, visit Wikipedia entry "Aliens" (motion picture), and see paragraphs devoted to weapons created for and seen in the film.

Strongly desired by Daily Show host Jon Stewart, who during a "Doom"-related TDS interview with The Rock, claimed that he "got a hard-on" after temporarily gripping a B.F.G.; The Rock verified that Stewart indeed did get an erection after holding the weapon. Such is the power of the B.F.G.. If a B.F.G. can give Jon Stewart a hard-on, you would be wise not fuck with it.
"You get in my face, I will so waste your hillbilly ass with this B.F.G.."

or

Seen used in "Aliens" by Private Vasquez immediately following the line, "LET'S ROCCCCCCK!"
by Brandywine September 19, 2006
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B.F.G
Stands for Based Fellatio Goat
Based is being biased in your opinion

Fellatio is oral stimulation or oral sex, typically fye
Goat is greatest of all time
Kiran: Hey is that the B.F.G?
Vik: Yep sure is...
Kiran: You staying active living up to your name playa?
Vik: Of course! Just got the top and off to hit a lick. Bye now boy and also fix that yee yee ass haircut lookin’ headass!
by BakaSenpaiBeanos January 11, 2021
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Bottom Floor Goons

In the comedy TV show (Danish) the group of friends who live on the bottom floor of a maisonette they share with a group of academic high-achievers who live on the top-floor. (Up Stairs Crew). The B.F.Gs was the name given to them by the USC in the second episode.

The TV show was originally aired on 21st September 2011.
Rose (B.F.Gs): "We're going out to dinner tonight"

Charlotte (USC): "Meehh thut thut, we got Buffalo Bills reserved for 3 instead, see you later!"

Shareen (BFG): "Ohh duuude..!"
by friedfrogg December 22, 2011
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Big F**kin' Gun(s)
He took out these B.F.G.s and went crazy.
by Chewy November 8, 2003
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Guy1: Hey, have you seen Dave's B.F.G?
Guy 2: Yea she's like 6 foot 5!
by vots69 February 1, 2010
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Big fragging gun(s) -- Either a specific gun in a video game by this name, or simply the biggest, baddest, mofo-est gun in a given game, e.g. the rocket launcher in Quake I.
Those guys were pwning me at Quake II until I got the B.F.G.(s) and started fragging them all over the place.
by PsyMar February 18, 2006
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