Green’s Dictionary of Slang

1. the lowest order and more contemptible species of thieves who lurk around and grab whatever they can regardless of value.

1753		J. Poulter Discoveries (1774) 37: I have known a Sneakman in the Morning, fetch down Stairs a Bed tied up in a Blanket, and meet the Master at the Door, who said, What have you got there? One of your Beds, Sir, replied the other, that Madam has sent to the Upholsterer.
1812		Vaux Vocab. of the Flash Lang.
1834		(con. 1737–9) W.H. Ainsworth Rookwood (1857) 178: Until at last there was none so knowing, / No such sneaksman or buzgloak going.
1851		Bell’s Life in Sydney 22 Mar. 2/6: [heading] A Sneaksman [...] committed to take his trial for sneaking into a public-house in Sussex street[...] and bearing away the contents of the till,.
1856		H. Mayhew Great World of London I 46: ‘Sneaksmen,’ or petty cowardly thieves [...] sneak off with either goods or animals.
1857		Wilmington jrnl (NC) 30 Jan. 1/4: Land of Thieves [...] Burt Miller is a ‘Hotel Sneaksman’.
1859		Hotten Dict. of Modern Sl. etc. 97: SNEAKSMAN, a shoplifter, a petty cowardly thief.
1861		H. Mayhew London Labour and London Poor IV 25: ‘Sneaksmen,’ or those who plunder by means of stealth.
1863		Sydney Morn. Herald 11 Aug. 2/4: The cracksman would no more think of associating with the sneaksman than a barrister with an attorney.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].
1882		Sydney Sl. Dict. (2 edn) 8: Sneaksmen - Petty larceny men and boys.

2. a shoplifter.

1859, 1864, 1870

see sense 1.