A sword. Bilboa in Spain was once famous for well-tempered blades: these are quoted by Falstaff, where he describes the manner in which he lay in the buck-basket. Bilboes, the stock; prison. Cant.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Bill at Sight * To BilkNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes