The last landed, inned, or stowed, of any sort of merchandise: so called by the water-side porters, carmen, &c. All the fat is in the fire; that is, it is all over with us: a saying used in case of any miscarriage or disappointment in an undertaking; an allusion to overturning the frying pan into the fire. Fat, among printers, means void spaces.
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.
Fastnesses * As Fat As a Hen In the ForeheadNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes