'non-isomeric' originates from the English prefix 'non-' (from Latin 'non'), meaning 'not', combined with 'isomeric', which ultimately comes from Greek 'isōmerēs' (via New Latin 'isomericus'), where 'isos' meant 'equal' and 'meros' meant 'part'.

'isomeric' changed from Greek 'isōmerēs' to New Latin 'isomericus' and then into modern English as 'isomeric'; the negative prefix 'non-' was later applied in modern English to form the compound adjective 'non-isomeric'.

Initially 'isōmerēs' meant 'equal in parts'; in chemistry 'isomeric' came to mean 'having the same molecular formula but a different arrangement', and 'non-isomeric' evolved to mean 'not isomeric' (not having the same molecular formula/arrangement).