'ill-defined' originates from the combination of 'ill' and 'defined', where 'ill' meant 'badly' and 'defined' meant 'clearly marked or outlined'.

'ill-defined' changed from the combination of 'ill' and 'defined' in Middle English and eventually became the modern English word 'ill-defined'.

Initially, it meant 'badly defined', but over time it evolved into its current meaning of 'not clearly or sharply defined'.