'splayed-footed' is a compound formed from 'splayed' (the past participle/adjective from 'splay') + 'footed' (from 'foot' + adjectival suffix '-ed'). 'splay' originates from Old French (via Middle English), specifically from words like 'espaler'/'espaillier' where the root meant 'to spread out' or 'to spread apart'; 'foot' originates from Old English 'fōt' meaning 'foot'.

'splay' entered English through Middle English from Old French related terms meaning 'to spread'; it produced the past form 'splayed'. Combined with the Old English-derived 'foot' plus the adjectival '-ed', the compound 'splayed-footed' developed in modern English to describe outward-turned feet.

Initially the elements conveyed the concrete sense 'spread' (for 'splay') and 'foot' (for 'foot'); over time the compound came to be used as an adjective specifically describing the condition of feet being turned outward, a meaning that closely follows the original components.