means A small unit of weight (about 28 grams in the avoirdupois system, or roughly 31 grams troy for precious metals), or by extension a tiny amount of anything.

from From Latin 'uncia,' meaning a twelfth part — the same word that gave us 'inch.' The Roman 'uncia' was one-twelfth of a 'libra' (pound) or of a 'pes' (foot), which is why the ounce and the inch are linguistic twins separated only by what they measure. It traveled through Old French 'unce' into Middle English. The 'oz' abbreviation, oddly, comes not from English but from the medieval Italian 'onza.'