A pilferer. Morning sneak; one who pilfers early in the morning, before it is light. Evening sneak; an evening pilferer. Upright sneak: one who steals pewter pots from the alehouse boys employed to collect them. To go upon the sneak; to steal into houses whose doors are carelessly left open. Cant.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Snatch Cly * SneakerNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes