Green’s Dictionary of Slang

1. one who pries, who is inquisitive; often applied to local government officials.

1891		Century Dict. 5733/2: snoop. [...] One who snoops, or pries or sneaks about; a snooper.
1912		J. London Smoke Bellew (1926) 92: Oh, you’ll get yours as soon as I finish with your pardner, you little hog-wallopin’ snooper, you.
1926		Dly News (NY) 8 Feb. 80/2: The snooper [...] scuttled up and down the aisle, sniffing the aroma of tobacco and watching for the telltale flares that signified that someone was igniting a pill.
1931		Hartlepool Nthn Dly Mail 19 Mar. 3/8: Federal dry agents, commonly known as ‘snoopers’, are about to big an aeronautic search for illicit whisky stills.
1937		F. Gruber ‘Death on Eagle’s Crag’ in Goulart (1967) 193: McClosky, the lousy old snooper, found the hypo in my room.
1941		K. Tennant Battlers 267: I don’t want none of them snoopers askin’ where you got Jimmy.
1952		Lait & Mortimer USA Confidential 1: The burgeoning bureaucrats send the same snoopers out to police gambling.
1960		C. Himes Big Gold Dream 115: If you spot any snoopers, point them out.
1978		F. Norman Dead Butler Caper 7: I’m an old-fashoined divorce snooper, process server, and all-round bearer of bad tidings.
1987		B. Chatwin Songlines 50: He banned [...] all anthropologists, journalists and other snoopers.
1998		G. Burn Happy Like Murderers 287: For most of 1976 the snoopers from the Council were still snooping around.
2000		J. Hawes Dead Long Enough 304: Which, according to Shnade [...] a fine authority on the British Social Security system, was the time we needed to make sure the Snoopers would have come and gone.

2. (also snooper hound) a private detective.

1916		J. Lait ‘‘Taxi, Mister!’’ in Beef, Iron and Wine (1917) 143: The merchant had had private snoopers shadowing his wife and one of them had taken Marty’s number.
1939		Howsley Argot: Dict. of Und. Sl.
1944		C.B. Davis Rebellion of Leo McGuire (1953) 42: He wasn’t a cop [...] He was a snooper.
1949		Monteleone Criminal Sl. (rev. edn) 216: snooper [...] snooper hound A detective; a plain clothes officer.
1958		R. Prather Scrambled Yeggs 14: How’s the head, snooper?
1974		‘P.B. Yuill’ Hazell Plays Solomon (1976) 38: I was only the doorstep snooper with the dodgy table manners.
1986		Cincinnati Enquirer (OH) 1 Feb. 45/4: Sledge [...] says he’s a P.I., a shamus, [...] a snooper, a peeper.

3. a thief.

1924

Galsworthy White Monkey 199: Her husband is a decent little snipe for a snooper.