Green’s Dictionary of Slang

slim adj.

(S.Afr./US/Aus.) clever, crafty, wily.

1804		J. Barrow Travels 103: A man who in his dealings can cheat his neighbour is considered as a slim mensch, a clever fellow.
1848		J.R. Lowell Biglow Papers (1880) 112: I wish I may be cust / Ef Bellers wuz n’t slim enough to say he would n’t trust!
1892		J.R. Couper Mixed Humanity 77: I am too slim for them, believe me.
1899		G.H. Russell Under the Sjambok 184: You ought to be called ‘Slim’ (sly) Piet, instead of ‘Long’ Piet.
1900		‘Doss Chiderdoss’ ‘Good in Their Different Lines’ Sporting Times 6 Jan. 1/4: She turned round and saw the ‘slim’ slop, who’d the bliss / To be good in his own line of business.
1900		B. Mitford Aletta 105: Adrian is slim.
1903		Truth (Sydney) 23 Aug. 3/3: Now, these [South Africans] are a rotton peepul— / Lyin’ thieves— they calls ’em ‘slim’.
1908		D. Blackburn Leaven 42: He knew how to be sick and lame when he would, so as to deceive even the white doctor [...] and was altogether a ‘slim’ kafir.
1911		Blackburn & Caddell Secret Service in S. Afr. 60: The Kaffir is a ludicrous failure at fraud, duplicity, and that low smartness which the Boers describe by the adjective ‘slim’.
1913		Truth (Sydney) 3 Aug. 12/4: When the Foreigner discusses / Of our ways, and calls ’em ‘slim,’ / I endorses of his judgment.
1941		C.R. Prance Tante Rebella and her Friends (1951) 117: That ‘slim trick’ was one more count in the republican indictment against Botha as a ‘Khaki General’, a traitor to the cause of freedom.
1941		C.R. Prance Riddle of the Veld 35: That ‘slim trick’ turned the Jew’s friendship into bitter hate.
1948		O. Walker Kaffirs are Lively 56: The third was a cattle speculator who was regarded by officials as a very ‘slim’ – cunning – bird.
1966		J. Bennett Hawk Alone 191: Don’t let them verneuk you, Mr Vance. [...] These gooses are damn slim, man. You got to watch them.
1973		P. Driscoll Wilby Conspiracy (1991) 52: He’s a bright boy, a slim kaffir.

In derivatives

slimness (n.)

craftiness, guile.

1902

Sun. Times (Perth) 1 June 4/6: He is an unregistered bookmaker [and] he naturally chooses a clerk whose reputation for ‘slimness’ is scarcely inferior to his own.