The story
Smithfield (a London meat market) + stone. Calculated on the basis that for every 14lb (1 standard stone) of livestock purchased, 8lb (1 Smithfield stone) of saleable meat are produced.
Etymology from Wiktionary, CC BY-SA 4.0.
Smithfield stone
Today it means: A British unit of weight used in the meat trade, equal to eight pounds (avoirdupois). Full definition →