'nonidentical-subunit' is a modern English compound formed from the negative prefix 'non-' + 'identical' + 'subunit'. 'non-' comes from English usage meaning 'not', 'identical' from Latin/Greek roots meaning 'the same', and 'subunit' from Latin 'sub-' (under, secondary) + 'unit' (a single entity).

'identical' derives from Latin 'identicus' (via French 'identique'), ultimately from Greek 'idem'/'id-' meaning 'the same'. 'subunit' is built from Latin 'sub-' + late Latin 'unitas' (unit). The compound 'nonidentical subunit' is a descriptive technical formation in modern scientific English combining these elements.

Originally the parts 'non-', 'identical' and 'subunit' retained their literal senses ('not' + 'the same' + 'component'); over time the compound stabilized as a technical term used especially in biology/biochemistry to denote heteromeric components in complexes.