The stone or stone weight is an English and imperial unit of mass now equal to 14 pounds. England and other Germanic-speaking countries of northern Europe formerly used various standardised “stones” for trade, with their values ranging from about 5 to 40 local pounds depending on the location and objects weighed

UK plural stone or stones (written abbreviation st) a unit of weight equal to 14 pounds or 6.35 kilograms, used especially when talking about a person’s weight: I weigh ten and a half stone. She has put on/lost a stone (= is a stone heavier/lighter). More examples. I want to lose a stone before Christmas.
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