Green’s Dictionary of Slang

1. (UK Und.) working as a sneak-thief.

1865		Leaves from Diary of Celebrated Burglar 85/1: That was no drawback to Granny Dixon’s ‘sneak’ hen – she gave a jump, and in a whiff she stood noiseless on the top edge of the half door.
1867		Nat. Police Gaz. (NY) 5 Oct. n.p.: Fred Louther [...] the sneak robber.
1886		A. Pinkerton Thirty Years a Detective 136: I will relate the incidents of a daring and successful robbery, by sneak thieves, which took place in the city of New York during the month of January, 1878.
1927		Clark & Eubank Lockstep and Corridor 174: Sneak work—house robbery while the inmates are at a meal.

2. (Aus./US) secret, surreptitious.

1968		K. Brasselle Cannibals 81: Her roommate was a sneak junkie.
1998		R.G. Barrett Mud Crab Boogie (2013) [ebook] ‘This [i.e. a boat] is his sneak go. Not many people know he owns it’.
2023		(con. 1962) J. Ellroy Enchanters 400: ‘Give me a sneak peak’.