Green’s Dictionary of Slang

firm n.

1. a criminal gang, large or small.

[UK]B.H. Malkin (trans.) Adventures of Gil Blas (1822) II 173: His funds were limited [...] I therefore was the monied man of the firm: but then there was brass in his forehead for an inexhaustible coinage.
[UK]Satirist (London) 8 Jan. 14/3: He sat down to Ecarté, with one of the ‘firm,’ to whom he lost within 801. of all he had before won.
[UK]Paul Pry 20 Sept. 178/8: Mr. Waller rose from the table [...] minus 63l. He has since been informed [...] that he had been robbed by the firm introducing loaded dice, by which it was actually impossible he could win.
Causes of the Increase of Crime in NY 27: [A]t various times, the captain, on moving from one precinct to another, appears to have taken his ward man along with him, and the same firm has set up business in a new sphere of operations.
[UK]Clarkson & Richardson Police! 221: The ‘firm’ – his own friends – keep filling up spaces.
[US]H.E. Hamblen Yarn of Bucko Mate 111: There was a big, swarthy villain among the players, who, I decided, was a member of the firm.
[US]W. Irwin Confessions of a Con Man 38: After monte had got in pretty bad repute, I became a member of the firm which rejuvenated it.
[UK]‘Sapper’ Bulldog Drummond 108: Is he one of the firm? [...] His face seems familiar.
[UK]L. Ortzen Down Donkey Row 32: ‘The old firm,’ said he, ‘always there ter take yer com-commis-bets, and always ’ere to pay yer out.’.
[UK]F. Norman Fings II i: On the firm / A four quid perm.
[UK]R. Cook Crust on its Uppers 28: The poor old firm was going a bit unsteady.
[UK]G.F. Newman Sir, You Bastard 25: If a firm was at it, then they would obviously wait for him to pass by.
[Aus]B. Ellem Doing Time 1289: firm: a group of criminals who work and do ‘jobs’ together.
[UK]S. Berkoff West in Decadence and Other Plays (1985) 123: He can stand up West and join the firm of grievous / rape / robbery and death / solicitors to the realm.
[UK]D. Campbell That Was Business, This Is Personal 3: [Armed robbers] had little to do with the old family ‘firms’ and little interest in owning clubs and casinos like the Krays.
[UK]J.J. Connolly Layer Cake 6: To a lot of firms around London it’s the one thing they have in common is a mutual respect for Morty.
[Aus]R.G. Barrett Mystery Bay Blues 12: She’d been trawled up with a firm who all finished with big sentences.
[UK]D. Seabrook Jack of Jumps (2007) 272: Teddy Smith was on the Firm, and yet he wasn’t really of the Firm.
[UK]J.J. Connolly Viva La Madness 59: He’s ringing it that it was his idea for Ted to get me back on the firm.

2. a group of people.

[US]Whip & Satirist of NY & Brooklyn (NY) 21 Jan. n.p.: The Editor of the Whip or ‘One of the Firm’ cares no more about them than they do about the idle wind.
[UK]Sporting Times 26 Jan. 1/3: Two of our young men drearily asking each other whether there was the price of a drink in the firm.
[UK]‘Bartimeus’ Long Trick 23: They gave me six months’ full pay sick leave — not a bad old firm, the Admiralty.

3. any form of gang, e.g. of football hooligans.

[US]O. Johnson Varmint 97: If you do us we’ll take you into the firm.
[UK]F. Slee diary 20 Oct. 🌐 We had some beer in a dixie and the steward found some bottles for our firm, who were always thirsty.
[UK]D. Powis Signs of Crime 183: Firm [...] humorously used to describe a particular squad of detective officers, especially a closely knit and comradely band.
[UK]R. Hewitt White Talk Black Talk 21: There were other ‘firms’ of youngsters who were not necessarily associated with any one pub.
[UK]K. Sampson Awaydays 84: The back-up firm can’t like what they see, because they disappear.
[UK]K. Sampson Outlaws (ms.) 8: Out jumps this little firm, hired in and that, tool-handed, proper mob and that.

4. an influential group.

[UK]Robins & Cohen Knuckle Sandwich 23: Long established local families [...] they were undoubtedly the ‘top firm’, at least in their part of the estate.

In phrases

get on the firm (v.)

to charm, to please, to seduce.

[UK] in G. Tremlett Little Legs 71: I tried to chat her up but couldn’t get on the firm.
odd-mark firm (n.)

(UK Und.) a group or gang that includes various different types of minor criminals, and even a few non-criminal types.

[UK]Guardian G2 10 Aug. 24: A pick and mix concoction of lesser criminals, minor criminals and even a couple of straight goers. What we call an odd-mark firm.
on the firm (adj.)

taking part, committed.

[Scot](con. 1980s) I. Welsh Skagboys 254: They got different customs arrangements for staff, and he got lads there that’s all on the firm.