'undertone' originates from English, specifically the combination of the prefix 'under' + 'tone', where 'under' meant 'below' or 'beneath' and 'tone' meant 'sound' or 'pitch'.

'under' comes from Old English 'under'; 'tone' comes via Old French 'ton' from Latin 'tonus' and Greek 'tonos'. The compound 'undertone' developed in English by combining these elements to mean a lower or underlying tone and later extended to subtler figurative senses.

Initially it referred to a lower or less prominent sound or pitch; over time it broadened to mean any subtle or underlying quality, implication, or hint.