uncia

Uncia is Latin for one-twelfth. The Roman pound, the libra, was divided into twelve unciae. The Roman foot was also divided into twelve unciae. The same word named both subdivisions because the Romans thought in twelfths: the concept was the fraction, not the thing being fractioned. When Latin passed into Old French, uncia became unce for weight and once for volume. English borrowed both.

The divergence happened slowly. Old French unce became Middle English unce, then ounce. The weight ounce settled at one-sixteenth of the avoirdupois pound — no longer one-twelfth of anything, because the English pound had switched to a sixteen-ounce system by the fourteenth century. The troy ounce, used for precious metals, retained a different weight: 31.1 grams versus the avoirdupois ounce at 28.35 grams. The fluid ounce added a third meaning entirely, measuring volume rather than weight.

Meanwhile, the same Latin uncia traveled a different path into English as 'inch.' From Latin uncia to Old English ynce to Middle English inch. One word in Latin became two words in English, and neither remembers the other. Nobody learning the word 'ounce' thinks of 'inch,' and nobody measuring an inch thinks of weight. The shared ancestry is invisible.

The ounce now has at least three active meanings in English: the avoirdupois ounce (28.35 grams), the troy ounce (31.1 grams), and the fluid ounce (29.57 ml in the US, 28.41 ml in the UK). Gold is sold by the troy ounce. Flour is sold by the avoirdupois ounce. Water is sold by the fluid ounce. One word, three entirely different quantities, all from a Latin fraction that meant one-twelfth of whatever you were dividing.