avoirdupois (n.) Look up avoirdupois at Dictionary.com
1650s, misspelling (with French du for de) of Middle English avoir-de-peise, the Norman form of Old French avoir de pois "goods of weight" (equivalent to Medieval Latin averia ponderis), from aveir "property, goods" (noun use of aveir "have," from Latin habere; see habit (n.)) + peis "weight," from Latin pensum, neuter of pendere "to weigh" (see pendant (n.)).

The oldest sense in English is "goods sold by weight" (early 14c.); from late 15c. as a system of weights in which 1 pound = 16 ounces. Introduced into England from Bayonne, from late 15c. it was the standard system of weights used in England for all goods except precious metals, precious stones, and medicine.