Green’s Dictionary of Slang

1. (Aus.) a character, an eccentric, one who never gives up despite any circumstances, also used of a horse; often intensified as hard doer under hard adj.

1857		Bell’s Life in Victoria (Melbourne) 29 Aug. 2/6: ‘I think this filly’s a good doer, for she guttles down her oats like a drunkard does his morning dram’.
1912		Table Talk (Melbourne) 7 Oct. 34/1: A great colt and a great ‘doer’ is Strathredole. / [...] / He’s a great ‘doer’ all right, but he won’t ‘do’ us any more.
1919		W.H. Downing Digger Dialects 19: doer (n.) — A person, usually humorous, reckless, undisciplined, immoral or eccentric.
1929		K.S. Prichard Coonardoo 300: You know Monty, damned old hard case, one of the hardest doers in the Nor’-West.
1930		L. Lower Here’s Luck 102: ‘That young Stanley must be a bit of a doer!’ ‘You said it,’ I agreed.
1938		D. Runyon ‘Lonely Heart’ in Runyon on Broadway (1954) 493: Moreover he is a pretty good doer, at that.
1943		J.A.W. Bennett ‘Eng. as it is Spoken in N.Z.’ in AS XVIII:2 Apr. 88: A doer is a jester, or an eccentric.
1951		D. Stivens Jimmy Brockett 140: Bit of a doer was young Herb, particularly with the little tarts. He did pretty well for himself, though most of ’em weren’t my meat. No style about them.
1959		A.W. Upfield Bony and the Mouse (1961) 69: ‘Hard doer, ain’t he?’ grumbled the council staff.

2. a cheat, one who defrauds another.

1920		Table Talk (Melbourne) 7 Oct. 34/1: A great colt and a great ‘doer’ is Strathredole. / He ‘did us’ for nearly a hundred on Saturday. / He’s a great ‘doer’ all right, but he won’t ‘do’ us any more.
1984		Partridge DSUE (8th edn) 322/2: from ca. 1840.

3. (US) a criminal.

1995		G. Pelecanos Down by the River 26: [of a murder] ‘The tire tracks indicate that the doers drove some kind of off-road vehicle’.
2000		personal correspondence 7 July: doer (US) criminal. Seattle police slang. My first sighting was in a 1999 novel by Ridley Pearson, with no suggestion that it was a new usage.