'unequal-leavedness' originates from modern English, specifically formed by combining the adjective 'unequal' + the past-participial adjective-forming element 'leaved' (from 'leaf') and the noun-forming suffix '-ness'.

'unequal' itself comes from Old French 'inegal' (later Middle English 'unequal'), ultimately from Latin 'inaequalis' where 'in-' meant 'not' and 'aequalis' meant 'equal'. 'Leaf' comes from Old English 'lēaf'. The full compound is a descriptive modern English formation rather than a single inherited root.

The formation has always been descriptive — originally components meant 'not equal' + 'leaf' + nominal '-ness'; the modern compound specifically denotes the botanical condition of leaves differing in size/shape.