'ill-proportioned' originates from a compound of Old English 'ill' (from Old English 'yfel') and Latin 'proportio' via Old French/Middle English, where 'ill' meant 'bad' or 'not' and 'proportio' meant 'a part' or 'measure'.

'proportio' (Latin) → Old French 'proportion' → Middle English 'proportion'/'proportioned'; the negative prefix 'ill-' (Old English) was later combined with the past-participial adjective 'proportioned' to form the compound 'ill-proportioned' used in Modern English.

Initially the elements referred to 'not in the correct share/measure'; over time the compound has retained this core sense, coming to mean generally 'poorly proportioned' in Modern English.