'one-toned' originates from Modern English compounding of the numeral 'one' and the adjective 'toned' (from 'tone' + '-ed'), where 'one' meant 'single' or 'unitary' and 'tone' referred to 'sound' or 'color/shade'.

'tone' itself comes from Latin 'tonus' via Old French and from Greek 'tonos' meaning 'stretching, pitch'. The compound 'one-toned' developed in recent English usage by combining 'one' + 'toned' to describe something having a single tone (sound or color); it emerged in the 19th–20th century as descriptive compound formation became common.

Initially the elements literally meant 'a single tone' (a single sound or shade); over time the compound came to be used more broadly and figuratively to describe monotony or lack of variation in voice, expression, or appearance.