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1650s, misspelling of Middle English avoir-de-peise (c.1300), from Old French avoir de pois "goods of weight," from aveir "property, goods" (noun use of aveir "have") + peis "weight," from Latin pensum, neuter of pendere "to weigh" (see pendant (n.)). After late 15c., the standard system of weights used in England for all goods except precious metals, precious stones, and medicine.
avoirdupois av·oir·du·pois (āv'ər-də-poiz')
Body weight; fat or fatness: I have too much avoirdupois, so I'm dieting