apothecaries' weight

noun
1.
a system of weights used chiefly in compounding and dispensing drugs: 20 grains = 1 scruple (℈); 3 scruples = 1 dram (dr); 8 drams = 1 ounce); 12 ounces = 1 pound. The grain, ounce, and pound are the same as in troy weight, the grain alone being the same as in avoirdupois weight.
Origin
1755-65
British Dictionary definitions for apothecaries' weight

apothecaries' weight

noun
1.
a system of weights, formerly used in pharmacy, based on the Troy ounce, which contains 480 grains. 1 grain is equal to 0.065 gram
apothecaries' weight in Medicine

apothecaries' weight n.
A system of weights used in compounding prescriptions and in which an ounce equals 480 grains and a pound equals 12 ounces.

apothecaries' weight in Science
apothecaries' weight
  (ə-pŏth'ĭ-kěr'ēz)   
A system of weights used in pharmacy and based on an ounce equal to 480 grains and a pound equal to 12 ounces. It has been largely replaced by measures of the metric system.
Encyclopedia Article for apothecaries' weight

traditional system of weight in the British Isles used for the measuring and dispensing of pharmaceutical items and based on the grain, scruple (20 grains), dram (3 scruples), ounce (8 drams), and pound (12 ounces). The apothecaries' grain is equal to the troy and avoirdupois grains and represents 15,760 part of the troy and apothecaries' pound and 17,000 part of the avoirdupois pound. One apothecaries' pound equals approximately 0.82 avoirdupois pound, 373.24 grams, and 0.37 kilogram.

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