Beggars, or highway spies, who traverse the road, to give intelligence of a booty; also rogues ready to snap up any booty that may offer, like privateers or pirates on a cruise.
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.
Crown Office * CrummyNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes