unwritten law

American  

noun

1. a law that rests for its authority on custom, judicial decision, etc., as distinguished from law originating in written command, statute, or decree.

2. the unwritten law, the supposed principle of the right of the individual to avenge wrongs against personal or family honor, especially in cases involving relations between the sexes: sometimes urged in justification of persons guilty of criminal acts of vengeance.

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unwritten law British  

noun

1. the law based upon custom, usage, and judicial decisions, as distinguished from the enactments of a legislature, orders or decrees in writing, etc

2. the tradition that a person may avenge any insult to family integrity, as used to justify criminal acts of vengeance

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unwritten law Idioms  

1. An accepted although informal rule of behavior, as in It's an unwritten law that you lock the gate when you leave the swimming pool. [Mid-1400s]

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Etymology

Origin of unwritten law

First recorded in 1635–45

Example Sentences

There’s an unwritten law that literary fiction set in the high plains be sturdy and simple — sentences firm as fence posts, commas hammered in as clean as barn nails.

From Los Angeles Times

But the facts of that night were swallowed up by a code of silence, the unwritten law that presumed police officers would shield one another from accountability.

From New York Times

For years, surfers, surf magazines and surf photographers mostly lived by that unwritten law in order to keep surfing’s secret spots secret.

From New York Times

The unwritten laws of baseball and common sense, which rarely coincide with each other on many situations, said it would happen eventually.

From Seattle Times

Even if there had been a good deli, unwritten laws said New York’s was best.

From Seattle Times