A highwayman. Royal scamp: a highwayman who robs civilly. Royal foot scamp; a footpad who behaves in like manner.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
scaly fish * to scamperNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes