'in tune' originates from English as a combination of the preposition 'in' and the noun 'tune'. 'Tune' ultimately comes from Latin 'tonus' (via Old French/Medieval Latin), where 'tonus' meant 'tone' or 'stretch/tension'.

'tune' changed from Old French/Medieval Latin forms (e.g. Old French 'ton', Medieval Latin 'tonus') into Middle English 'tune' and eventually became the modern English 'tune'. The phrase 'in tune' developed as an idiomatic collocation meaning 'in the same tone' and then extended metaphorically to 'in agreement'.

Initially, it meant 'in the same tone' (literal musical sense), but over time it evolved to include the current broader meanings of 'harmonious, in agreement, or attuned'.