'serpent-legged' originates from Modern English, a compound of the noun 'serpent' (from Latin 'serpens', where 'serp-' meant 'to creep' or 'creeping thing') and the adjective-forming element '-legged' (from Old English 'lǣg'/'leg' meaning 'leg' plus the participial/adjectival suffix '-ed').

'serpent' entered English via Old French 'serpent' from Latin 'serpens'. The element '-legged' arose in English by combining 'leg' with the suffix '-ed' to form descriptive adjectives (cf. 'cloven-footed', 'winged'), and the compound 'serpent-legged' is a later descriptive formation in Modern English used especially in literary and mythological contexts.

Initially the parts referred separately to 'serpent' and 'leg'; when combined into 'serpent-legged' the phrase came to mean 'having legs like a serpent' and to carry connotations of mythic or monstrous imagery rather than a purely literal anatomical description.