'Ounce' and 'inch' are the same Latin word — 'uncia' (a twelfth) — split into weight and length.
A unit of weight equal to one-sixteenth of an avoirdupois pound (approximately 28.35 grams), or one-twelfth of a troy pound (approximately 31.1 grams).
From Middle English 'unce,' from Old French 'unce,' from Latin 'ūncia' (a twelfth part), from 'ūnus' (one). In the Roman system, the 'ūncia' was one-twelfth of a 'lībra' (pound) and one-twelfth of a 'pēs' (foot). The same Latin word also produced English 'inch
A troy ounce (31.1 grams, used for gold and silver) is heavier than an avoirdupois ounce (28.35 grams, used for everything else), but a troy pound (12 troy ounces = 373.2g) is lighter than an avoirdupois pound (16 avoirdupois ounces = 453.6g). The abbreviation 'oz.' comes from medieval Italian 'onza,' itself from Latin 'ūncia.'