shamus, n. — Green’s Dictionary of Slang

shamus n.

also sham, shamez, shammos, shamos, shom, shommus, shomus

[pop. attrib. to Seamus, the common Irish name of many policemen; but note Heb./Yid. shames, a synagogue official; for detailed study and verdict promoting the Heb. origin, see A. Manaster Ramer https://www.academia.edu/118986699/A_Joke_OK_But_What_WAS_So_Funny_Am_Engl_shamus_originally_policeman_later_private_detective_rare_burglars_jimmy_1_2]

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

1928		J. O’Connor Broadway Racketeers 181: It’s a tough racket now-a-days. The Shommuses have made the good spots too hot.
1929		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).
1930		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.
1931		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.’.
1933		Ersine Und. and Prison Sl. 66: shom, shomus, n. (From the Hebrew.) A cop, bull.
1942		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.
1955		‘Ed Lacy’ Best that Ever Did It (1957) 78: What’s with your great big birdbrain, shamus?
1977		D. Powis Signs of Crime 200: Shamez Synagogue official used as a slang word for policeman (Yiddish).
1989		J. Morton Lowspeak 126: Sham – a policeman [...] Shamez (Yiddish) a policeman.

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

1934		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.
1946		J. Evans Halo in Blood (1988) 19: Who was your friend, shamus?
1951		S.J. Perelman in New Yorker 3 Mar. 26/3: Scores of hoodlums, gunsels, informers, shyster lawyers and crooked shamuses.
1960		Wodehouse Jeeves in the Offing 39: You mean that I’m to be a sort of private eye or shamus.
1963		A.S. Fleischman Venetian Blonde (2006) 212: Who saw us on the beach? That cheap shamus?
1973		Kingston Dly Freeman (NY) 23 Feb. 43/1: Burt Reynolds [...] portrays a shamus (super tough private eye).
1987		R. Campbell Alice in La-La Land (1999) 133: This kid, looks like a girl, is a private eye. A shamus. A gum shoe.
1994		Baltimore Sun (MD) 23 Nov. 50/5: Like so many disgraced cops before, he’s hung out a shamus shingle.
2004		T. Dorsey Cadillac Beach 258: Should have strapped iron and put daylight into the hinky shamus who dropped the dime.
2013		T. Pluck ‘Six Feet Under God’ in Life During Wartime (2018) 190: ‘Amscray, shamus’.

3. (US und.) a patron, a protector, a ‘rabbi’.

1959

(con. 1900-29) L. Katcher Big Bankroll 322: Hines was McManus’ ‘shammus,’ his protector, his link with Tammany Hall.

4. a police informer.

1960, 1975

Wentworth & Flexner DAS.