A man who was hanged is said to have died of a hempen fever; and, in Dorsetshire, to have been stabbed with a Bridport dagger; Bridport being a place famous for manufacturing hemp into cords.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
hemp * hempen widowNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes