petra en · NOUN
Etymology
From Latin petra (“rock”). Doublet of piedra.
Meanings
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stone, a weight equal to 14 pounds.
Generally, however, the stone or petra, almost always of 14 lbs., is used, the tod of 28 lbs., and the sack of thirteen stone.
1882, James Edwin Thorold Rogers, A History of Agriculture and Prices in England, volume 4, page 209: