unwritten law

unwritten law idiom
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]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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