Green’s Dictionary of Slang

1. a false accusation or an unfair sentence.

1914		Amer. Mag. 12/3: I knew the coppers were working a ‘bum rap,’ for Mac had been with me all night.
1916		Non-Partisan Leader (Fargo, ND) 12 Oct. 18/2: He was incensed at the Fargo department because [...] they gave him a ‘bum rap’ in this matter of the revolver.
1922		Wash. Herald (DC) 11 Aug. 3/3: ‘Boys,’ he said to the reporters, ‘when this daffy trial started, I told you thi was a bum rap’.
1927		Clark & Eubank Lockstep and Corridor 173: Bum rap—a false charge.
1933		‘Goat’ Laven Rough Stuff 93: They tried their best to put some bum raps, that is crimes done by these wandering bums, on me.
1937		(con. 1905–25) E.H. Sutherland Professional Thief (1956) 119: Unquestionably a real thief would rather have an arrest on a good rap than on a bum one. When the rap is right, he knows what to do and how to go about it. He knows whom he must square. But in a bum rap he is entirely at sea.
1949		I. Shulman Cry Tough! 30: Also, If I’m working for a guy, not to take his bum raps.
1952		C. Hamilton Men of the Und. 127: It’s a bum rap.
1960		I. Freeman Out of the Burning (1961) 13: They busted me on a bum rap anyway. I was just sitting in a stolen car. I didn’t steal it.
1975		E. Torres Carlito’s Way 43: I got a bum rap on account of a stool pigeon name of Joe Ugatz.
1988		Viz Oct./Nov. 27: It’s a bum rap.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 182: There is a bloke in WA who thinks he [...] has been given life on the bum rap of shooting some two-bob bikie in Launceston.
2002		Fabian & Byrne Out of Time (ms.) 199: Tell her I’m the fall guy. I’ve had a bum rap.

2. (US prison) an inmate who proclaims themselves innocent.

1925

Institution Qly (IL) 16 129: An innocent man in the penitentiary is a ‘bum rap’.

3. a misfortune, an unfair action.

1924		Medical Herald 43-44 19/2: The fates gave him a bum rap .
1942		N. Algren Never Come Morning (1988) 185: That’s one bum rap can’t nobody beat.
1963		A.S. Fleischman Venetian Blonde (2006) 219: It’s kind of a bum rap to let her think she drove you to it.

4. harsh (and poss. unfair) criticism.

1952

W. Brown Sox 75: Maybe it was a bum rap for Heinie Zimmerman, but he wasn’t the first ball player [...] who had to take such, and like it [HDAS].

5. a bad reputation.

1977

C. McFadden Serial 73: Wolves are far out; they just get a bum rap.