Green’s Dictionary of Slang

G adj.1

[i.e. government]

(US) Federal.

implied in G-man
[UK]P. Cheyney Dames Don’t Care (1960) 29: I send a code wire to the ‘G’ office in new York.
[US]N. Algren Man with the Golden Arm 276: This is a G offence.
[US]Goldin et al. DAUL 82/2: G-joint [...] A federal penal institution.
[US]K. Brasselle Cannibals 62: The G attorney was too smart to get trapped.

In compounds

G-girl (n.) (also G-gal)

(US) a female Government employee.

[US]Lait & Mortimer USA Confidential 38: Females in farm houses, factories, furnished rooms and in furs are as promiscuous and predatory as the G-gals in the D.C.
A.F. Moe ‘Gyrene’ in AS XXXVII:3 178: G-girl ‘government girl,’ a term commonly applied during the 1930s to a woman employee of the Federal government.
G-guy (n.)

(US) an agent of the FBI .

[US]D. Runyon ‘Dream Street Rose’ in Runyon on Broadway (1954) 45: A bunch of G-guys step into the joint and bust it wide open.
Palm Beach Post (FL) 4 June 1/5: [headline] Davis, Member of Karpis Gang, to Get Life Term [...] ‘No Use Trying to Beat These G-Guys’ He Announces.
Palm Beach Post (FL) 29 July 20/4: The G-Guys had just caught six super-trained Hitler saboteurs.
G-heat (n.) [heat n. (3a)]

(US) trouble from, or agents of, federal law enforcement agencies.

[US]J.E. Hoover Persons in Hiding 114: Stay away from us. You’re hot. It’s G heat! We don’t want to lay eyes on you!
[US]S. Longstreet Decade 317: Nails Flowers is on the lam. They hung a rap on him at last. He chalked out a momser, and the coppers and harness bulls and G-heat are on him.
[US]E. Hunter ‘See Him Die’ in Jungle Kids (1967) 102: Also he was getting G-heat because the word was he transported some broads into Connecticut for the purpose of prostitution.
Longstreet & Godoff Remember William Kite 277: No G-heat on you?
G-man (n.) (also G, the) [note pre-US use as a political detective in pre-Independence Ireland]

1. (US, also G) an FBI agent, lit. ‘Government-man’.

[US]Hostetter & Beesley It’s a Racket! 225: G MAN—United States government employee, especially one in the secret service or similar police work.
[US]F.D. Pasley Al Capone 33: He offered a G man [government agent] ten gran’ to forget it.
[US]R.E. Sherwood Idiot’s Delight 150: I was in the Club Grotto the night the Purple Gang shot it out with the G’s.
[US] in W. Winchell ‘On Broadway’ 10 Aug. [synd. col.] [Hoover] has pet writers [...] who scratch his back in return for material that glorifies Edgar Hoover and the G’s.
[US]J. Archibald ‘No Place Like Homicide’ in Popular Detective Apr. 🌐 The Gs were after a firm down on Pearl Street for evading the bee that Uncle Sam puts on citizens.
[US]W.R. Burnett High Sierra in Four Novels (1984) 409: The big brave G-men! It took forty of ’em to knock Johnny [i.e. Dillinger] off.
[US]T. Runyon In For Life 52: The G-men [...] learned the only new fact the police ever got.
[US]K. Marlowe Mr Madam (1967) 35: When they’d make us write compositions about our fathers I’d always write that he was a G-man.
[US]Cab Calloway Of Minnie the Moocher and Me 101: Winchell’s guests were J. Edgar Hoover and Clyde Tolson, the G-men.
[US]Ice-T ‘Grand Larceny’ 🎵 Calling the police, calling the G-men, calling all Americans to war on the underworld.
[US]C. Hiaasen Native Tongue 250: Sharpshooter Widow Gunned Me Down, Nude G-Man Claims.
[US]D.H. Sterry Chicken (2003) 162: I had a minimal panic seizure. G-men in closets. SWAT team swooping down and busting my ass.

2. (S.Afr.) the vice and liquor squad.

[SA]Sophiatown in M. Orkin At the Junction (1995) 182: Oh my God! Dis die G-men! Vice en liquor squad!

3. (Aus.) a member of the federal (rather than state) police force.

[Aus]R.G. Barrett Wind & Monkey (2013) [ebook] They were federal cops. Australia’s version of the FBI. [...] Would be G-men, out for the glory and the elusive, multi-million dollar drug bust.
G-warning (n.)

(US gay) a warning that a club is due to be raided.

[US]Homosexuality & Citizenship in Florida 25: Glossary of Homosexual Terms [...] g-warning-g (general) red: tip-off on an oncoming raid which will happen soon. Warning for all gay people to leave the bars and head home [...] g-warning-local: Tip-off that the club in which it is given is about to be raided. Also, used to inform another person that you’re being watched and to be cautious.