'in tune' is formed from the preposition 'in' + the noun 'tune'. 'Tune' originates from Old French 'ton' (from Latin 'tonus') where 'tonus' meant 'tone, sound'.

'tune' came into Middle English as 'tune' (from Old French 'ton/tonus' and Latin 'tonus'), and the phrase 'in tune' has been used since early modern English to mean 'in the correct pitch' and later extended to figurative senses.

Initially, 'in tune' primarily meant 'in the correct musical pitch'; over time it evolved to include figurative senses of 'in agreement' or 'in harmony' with people, ideas, or circumstances.