means A unit of length equal to one twelfth of a foot, roughly the width of an adult thumb.

from From Old English 'ynce,' borrowed from Latin 'uncia' — meaning 'a twelfth part,' literally a small unit. The very same Latin 'uncia' also gave us 'ounce,' so the inch and the ounce are linguistic siblings, both descended from the Roman idea of a twelfth slice of a whole. The thumb connection is real too: many languages measure an inch by the thumb (Latin 'pollex' meant both thumb and inch), and the old definition pegged it to thumb-width, which is why 'inch' and the idea of a thumb keep shaking hands across the centuries.