a unit of weight equal to one sixteenth of a pound (about 28.35g)
a unit of troy weight or apothecaries’ weight equal to one twelfth of a pound or 480 grains (about 31g)
= fluid ounce
(usu with a negative) the least amount
The Queen is the prescriptive sovereign of one realm only, the United Kingdom: elsewhere neither she nor her so-called representatives the governors-general possess an ounce of prescriptive sovereignty — Enoch Powell
[Middle English via French from Latin uncia twelfth part, ounce, from unus one]