Green’s Dictionary of Slang

1. (US) a plain-clothes detective.

1849		Keowee Courier (Pickens, SC) 25 Dec. 1/5: Soon after his arrival [...] the shadow followed him. [...] He walked about the town [...] the shadow was behind him.
1852		N.Y. Times 30 Nov. 1/6: Officers Hogan and Clark, of the Chief’s ‘shadows’.
1869		N.Y. Times 19 May 5/1: It is the duty of the ‘shadow’ to follow him in, if he can do so without being ‘dropped on,’ (noticed by the clerk), to see how much money he spends, if any.
1871		Galaxy (N.Y.) Mar. 188: That is the room where the ferrets of the house who assume the name of detectives, but are more significantly called ‘shadows,’ are hidden from the prying eyes of the world. [Ibid.] 190: [...] That is to say, the husband’s man informs the wife she is watched, and gives her a minute description of her ‘shadow,’ for which information he of course gets an adequate reward, which the wife’s man likewise earns and receives by doing the same kindly office for the husband.
1886		‘Lady Kate, the Dashing Female Detective’ in Roberts et al. Old Sleuth’s Freaky Female Detectives (1990) 24/1: ‘You’re late, “pard”.’ ‘Had a shadow over me.’ ‘Did you drop it?’ ‘Yes.’.
1900		Chanute Times (KS) 12 Oct. 3/4: When a criminal loses his shadow the detective is apt to be puzzled.
1906		Cameron Co. Press (PA) 29 Mar. 6/3: A ‘shadow’ was placed on the house and its occupants .
1908		Queenslander 22 Feb. 44/4: One of the cleverest ‘shadows’ was put on his trail.
1914		Eve. Star (Wash., DC) 13 Dec. 47/1: The ‘shadow’ [...] is one of the most important and least written of personalities in the sphere of crime detection.
1929		Hostetter & Beesley It’s a Racket! 237: shadow—A detective, or a criminal’s assistant, who follows a suspect, or a prospective victim, to learn his movements and habits.
1931		Irwin Amer. Tramp and Und. Sl. 166: Shadow.–One who follows or keeps under surveillance; usually applied to a police officer or detective.
1949		A. Hynd We Are the Public Enemies 153: The shadow got into a second taxi.
1962		Ragen & Finston World’s Toughest Prison 817: shadow – One who follows or keeps under surveillance.
1984		W.J. Caunitz One Police Plaza 61: Braxton has a shadow.

2. (UK Und.) a pimp, male or female.

1876

J. Greenwood Low-Life Deeps 63: The shadder. That was the shadder, that woman that was with her [...] They’re no more their own mistresses than galley-slaves are.

3. (US) a derog. term for a black person.

1941		C.S. Johnson Growing Up in the Black Belt 259: Black youth are called by such derisive names as ‘Snow,’ ‘Gold Dust Boys,’ ‘Blue Gums,’ ‘Midnight,’ ‘Shadow,’ ‘Haint,’ ‘Dusty,’ ‘Polish,’ and ‘Shine.’.
1975		G. Underwood ‘Razorback Sl.’ in AS L:1/2 65: shadown Black (derogatory, white use).
1983		I.L. Allen Lang. of Ethnic Conflict 47: Color Allusions, Other than ‘Black’ and ‘Negro’: […] shadow.