Green’s Dictionary of Slang

an unreliable, evasive, untrustworthy person; often as nickname.

1797		M. Leeson Memoirs (1995) III 213: A Crupper-making squire I certainly am, / With my old auctioneer called slippery Sam. / With my haily gaily, cheat away daily.
1843		Cork Examiner 15 Mar. 4/5: Resembling a pick-pocket and being remanded [...] till your friends can [...] prove you are not Flash jack, alias Bunkem, alias the Mizzler, alias Jockey Wide O, alias Slippery Joe [...] alias Conkey Dick.
1854		Wilts. & Gloucs. Standard 9 Sept. 8/4: What must be said of the other son, [...] ‘Slippery Sam’ himself [...] who is a most unsafe and unstable guide.
1877		Star Guernsey 14 Aug. 2/4: A certain swindler known as ‘Slippery Sam’.
1887		South Wales Echo 13 May 4/4: Slippery Sam and Black Eli were arrested for fighting.
1900		Davenport Wkly Leader (IA) 17 Apr. 5/3: ‘Look here, Slippery Sam, you cannot play any double game with me, my man’.
[1919		Dundee Courier 21 Nov. 5/2: Mr Bailey described how a game called ‘Slippery Sam’ was played at the White Lodge].
1917		Coventry Herald 6 Jan. 2/4: Slippery sam lloked as if he meant to put his threat into execution.
1920		Eve, Teleg. (Angus, Scot.) 5 Jan. 4/4: Enough of Slippery Sam and his fortunes and misfortunes.
1920		Leics. Post 30 Dec. 5/6: A Slippery sam [...] an American who lies under arrest [...] on charges of swingling.
1926		Britannia & Eve (London) 23 June 36/1: A modern crook — Slippery Sam, say — would take about ten minutes to open it [i.e a safe].
1949		Dundee Eve. Teleg. 4 Nov. 8/2: [cartoon caption] Out shot Slippery Sam. His bag burst open and precious jewels showered out.
1950		Post 14 May 7/1: [of a fox] They’re on Look-Out for Slippery same. Dusk and dawn are the fox’s favourite hunting times [...] But Slippery Sam comes and goes unscathed.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 70: I said he was known as a slippery sam when it came to paying up.