/ˈdjuːti/
[noun] Something that you are expected or required to do because it is morally right, part of your job, or required by law. It can also refer to a tax paid on goods that are imported or exported.
The word 'duty' is rooted in the idea of a debt — your duty to someone was literally something you owed them.
The word 'duty' arrives in English in the late 14th century, borrowed from the Anglo-French 'duté' and Old French 'dueté', meaning 'obligation' or 'what is owed'. These Old French forms descended from the Old French adjective 'deu', meaning 'owed' or 'due', which itself came from the Latin 'debitus', the past participle of 'debere' — to owe, to be under obligation. At its very heart, then, 'duty' is a word about debt.
Latin 'debere' is a compound built from 'de-' (away) and 'habere' (to have), so its original sense was something like 'to have something taken away', meaning to have given something to another and therefore to be in a position of owing. From this same Latin root, English later received the words 'debt' and 'debit' — both following a more direct path through Old French 'dette' — meaning that duty, debt, and debit are all etymological cousins, each carrying the memory of something owed. The chain runs: Latin 'debere' → Old French 'deu' → Anglo-French 'duté' → Middle English 'duete' → modern English 'duty'.
When 'duty' first appeared in English texts, it carried a very strong moral and social weight. In a medieval feudal society, a person's duties were not abstract ideas but concrete, enforceable obligations — a knight owed military service to his lord, a serf owed labour to the manor, and a child owed obedience to a parent. The word described the entire web of social bonds that held society together. This is why 'duty' so quickly became connected to honour, virtue, and moral seriousness, rather than simply describing a task on a list.
By the 16th and 17th centuries, the meaning began to expand in interesting directions. 'Duty' took on a financial sense — a customs duty was a fee owed to the crown or state for the privilege of moving goods across a border. This usage kept the original 'owed' meaning very much alive, but applied it to commerce rather than personal honour. It was also during this era that philosophers began to build entire moral systems around the concept, most famously Immanuel Kant in the 18th century, whose 'duty ethics' — or deontology — argued that moral behaviour was defined entirely by fulfilling one's duties, regardless of the consequences.
The military sense of 'duty' — as in 'on duty' or 'active duty' — developed naturally from the medieval notion of obligatory service. A soldier on duty is quite literally fulfilling the obligation they owe to their country or commanding officer. This usage became widespread in the 17th and 18th centuries as professional standing armies replaced the old feudal levies. Over time, this phrase escaped the purely military context and entered everyday life: a nurse on duty, a police officer on duty, even a parent on night duty — all of them, in the oldest sense of the word, paying back what they owe.