Green’s Dictionary of Slang

shamus n.

also sham, shamez, shammos, shamos, shom, shommus, shomus
[Seamus, the common Irish name of many policemen; but note Heb./Yid. shames, a synagogue official]

1. (US) a police officer, usu. male.

[US]J. O’Connor Broadway Racketeers 181: It’s a tough racket now-a-days. The Shommuses have made the good spots too hot.
[US]S. Ornitz Haunch Paunch and Jowl 9: ‘A shammos! A shammos!’ (Synagogue beadle. In the secret lingo of the gang shammos was the warning that a policeman was coming).
[US]S.J. Perelman letter 31 Oct. in Crowther Don’t Tread on Me (1987) 7: ‘Oke, I’ll stand here and guard the hat while you run and get a cage,’ offers the shommus.
[US]Irwin Amer. Tramp and Und. Sl. 167: Sham.–A policeman, probably from the fact the majority of these men were from Ireland, and wearers of the shamrock. Shamos.–See ‘sham.’ Shamus.–See ‘sham.’.
[US]Ersine Und. and Prison Sl. 66: shom, shomus, n. (From the Hebrew.) A cop, bull.
[US]R.L. Bellem ‘Million Buck Snatch!’ Dan Turner - Hollywood Detective Jan. 🌐 A harness copper came bouncing into the joint. I stood up. The shamus spotted me.
[US]‘Ed Lacy’ Best that Ever Did It (1957) 78: What’s with your great big birdbrain, shamus?
[UK]D. Powis Signs of Crime 200: Shamez Synagogue official used as a slang word for policeman (Yiddish).
[UK]J. Morton Lowspeak 126: Sham – a policeman [...] Shamez (Yiddish) a policeman.

2. a detective, esp. a private operative; also attrib.

[US]R. Chandler ‘Finger Man’ in Pearls Are a Nuisance (1964) 73: ‘I think you are a dick. A smart dick.’ ‘Just a shamus,’ I said.
[US]J. Evans Halo in Blood (1988) 19: Who was your friend, shamus?
[US] S.J. Perelman in New Yorker 3 Mar. 26/3: Scores of hoodlums, gunsels, informers, shyster lawyers and crooked shamuses.
[UK]Wodehouse Jeeves in the Offing 39: You mean that I’m to be a sort of private eye or shamus.
[US]A.S. Fleischman Venetian Blonde (2006) 212: Who saw us on the beach? That cheap shamus?
Kingston Dly Freeman(NY) 23 Feb. 43/1: Burt Reynolds [...] portrays a shamus (super tough private eye).
[US]R. Campbell Alice in La-La Land (1999) 133: This kid, looks like a girl, is a private eye. A shamus. A gum shoe.
[US]Baltimore Sun (MD) 23 Nov. 50/5: Like so many disgraced cops before, he’s hung out a shamus shingle.
[US]T. Dorsey Cadillac Beach 258: Should have strapped iron and put daylight into the hinky shamus who dropped the dime.
[US]T. Pluck ‘Six Feet Under God’ in Life During Wartime (2018) 190: ‘Amscray, shamus’.

3. a police informer.

[US]Wentworth & Flexner DAS.

4. any person, also as an affectionate term of address; cites 2017 suggests use as a villain, the opposite of a police officer.

[US]S. Longstreet Decade 317: Cokey too, this shamos, and ready to kill on any beef.
[Ire]B. Behan Scarperer (1966) 11: All the shams, they put in threepence of a night with Pig’s Eye O’Donnell.
[Ire](con. 1930s) J. Healy Death of an Irish Town 20: They were, in the current slang of the town, ‘the buff shams’.
[Ire]T. Murphy Conversations on a Homecoming (1986) 54: No bother there, sham.
[Ire]D. Healy Sudden Times 14: Hi Sham, he says.
[Ire]G. Coughlan Everyday Eng. and Sl. 🌐 Sham (n): used by a man from a rural area when addressing one from the city e.g. How’s it goin’, sham?
[Ire]L. McInerney Blood Miracles 8: Half of Ireland will want in on it. The Shades and the shams, all looking for a cut [of drug dealing].
[Ire]L. McInerney Blood Miracles : The two oldest Cusack boys, a right pair of shams-in training, you’d want to givem a wide berth.
[Ire]L. McInerney Rules of Revelation 52: ‘Happened to a sham I used to pull pints with in Galway. He broke his hand in a fight’.