'ill-formed' originates from the combination of 'ill,' meaning 'badly,' and 'formed,' meaning 'shaped or structured.'

'ill-formed' evolved from the Middle English use of 'ill' as a prefix to denote something negative or bad, combined with 'formed' from Old English 'geformian,' meaning 'to shape or create.'

Initially, it meant 'badly shaped or structured,' and this meaning has largely remained the same in modern usage.