Rogues who, in conjunction with watermen, robbed, and sometimes murdered, on the water, by picking a quarrel with the passengers in a boat, boarding it, plundering, stripping, and throwing them overboard, &c. A species of badger. 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.
Ark * Arrah NowNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes