Something that you must do because of a rule, law, or strong expectation; required or necessary.
From Latin 'obligatorius,' derived from 'obligare' (to bind or commit). Entered English in the 1600s through legal and formal contexts, relating to being 'obliged' to do something.
The root 'obligare' literally means 'to bind'—so obligatory rules are things that bind you, that you're legally or socially tied to. It's why we still say 'I'm obliged to you' when someone does us a favor (you're bound by gratitude).
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