Duty is suffixed from due, which in turn is the English derivative from Latin's debitum, owed. Which is to say, duty is the state of being in debt. One's duty, is what one owes. But duty in the sense we now understand the term is disinterested, whereas debt as we now understand it (which is to say, now that usury has been stricken from the list of appalling sins by the Western world) is precisely interested ... interest is due upon debt: interest, we might say, is the duty of debt.
This semantic separation figures a more profound shift of cultural meaning than is generally understood. We need, if the term is to be made fresh and socially valuable again (as it really, really needs to be)--we need to remove 'duty' from the semantic field of debt. Our sense of duty is too lamentably that of something we owe, the realm of the 'owed' or 'ought' (hence we say: 'I ought to do more about global warming, I ought to give more money to charity'); but the sorts of things we owe now (our mortgage, for instance) are things we seek primarily to get rid of. Duty, in the broadest sense, cannot be 'discharged'; we can never be, and should not seek to be, in a position where all our 'duty' is paid off, and we can relapse into destructive selfishness. Duty is a freedom, not an obligation: a freedom from the tyranny of self, not a mortgaging of that self to society as a whole. Duty is always a free choice, a flowing-out of the human from ourselves to others. Duty is a liberality.