A set of rogues who are employed to look out and watch upon the roads, at inns, &c. in order to carry information to their respective gangs, of a booty in prospect.
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.
To Carouse * Carriers-1Nathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes