Avoirdupois \av-uhr-duh-POIZ\ or \AV-uhr-duh-poiz\ , noun;
1. Avoirdupois weight, a system of weights based on a pound containing 16 ounces or 7,000 grains (453.59 grams)
2. Weight; heaviness; as, a person of much avoirdupois
This word dates to the 1650's as a misspelling of Middle English avoir-de-peise which dates to the 1300's. Originally it derives from Old French avoir de pois ("goods of weight") from aveir + peis ("property, goods" + "weight"). Peis comes from Latin pensum, the neuter of pendere ("to weigh") from Proto-Indo-European *(s)pen(d)- ("to pull, stretch").
By the early 16th century avoirdupois was the standard system of weights used in England for all goods except precious metals, precious stones, and medicine. Eventually it was partially replaced by the International System of Units, but some remnants are still in daily use (i.e. the stone is 14lbs). It is still the system of weights (mass) we use in the United States based on the pound made up of 16 ounces.
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Monday, May 30
Etymologies come from the Oxford English Dictionary and/or Etymonline.com