'unequal-legged' is a compound formed from 'unequal' (from Latin 'inæqualis' via Old French 'inegal' and Middle English 'unequal', where elements conveyed 'not' + 'equal') and 'legged' (derived from 'leg', Old English 'lǣg/leg' with the adjective-forming '-ed' meaning 'having legs').

'unequal' entered Middle English from Old French 'inegal' (from Latin 'inæqualis'). 'Legged' is formed in Modern English by combining the noun 'leg' (Old English) with the suffix '-ed' to create an adjective meaning 'having legs'. These components were later combined into the descriptive compound 'unequal-legged'.

The elements originally meant 'not equal' and 'having legs'; together they evolved into the compound meaning 'having legs of unequal length', a specific descriptive usage in modern English.