'equal-legged' originates from modern English as a compound of 'equal' and 'legged'. 'equal' ultimately comes from Latin 'aequalis', where 'aequ-' meant 'level, even', and 'legged' is formed from English 'leg' plus the adjectival suffix '-ed'.

'equal' came into English via Old French and Middle English from Latin 'aequalis'; 'leg' comes from Old English 'leg' (related to Old Norse 'leggr'), and the adjective form with '-ed' developed in English to mean 'having legs'. The compound 'equal-legged' formed by combining these elements in modern English.

Initially the components meant 'even/level' (from 'aequalis') and 'leg', and when combined the phrase meant 'having legs of equal length'; this specific descriptive meaning has been retained in modern usage.