The highway, or a robber thereon; also a bed. Footpads; foot robbers. To go out upon the pad; to go out in order to commit a robbery.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1721 dictionary of canting and thieving slang.
packthread * pad borrowersNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes