'duty-bound' originates from the combination of 'duty' and 'bound', where 'duty' meant 'a moral or legal obligation' and 'bound' meant 'obligated or tied'.

'Duty-bound' evolved from the Middle English word 'bounden', which meant 'obligated', and was combined with 'duty' to form the modern term.

Initially, it meant 'obligated by duty', and this meaning has largely remained the same in modern usage.