gentlemen's agreement

WordReference Random House Unabridged Dictionary of American English © 2025

gen′tlemen's agree′ment, 

1. Sociology an agreement that, although unenforceable at law, is binding as a matter of personal honor.
2. Sociology an unwritten agreement by a socially prominent clique, private club, etc., to discriminate against or refuse to accept members of certain religious, racial, national, or other groups.
Also, gen′tleman's agree′ment. 
• 1885–90, American.

Collins Concise English Dictionary © HarperCollins Publishers::

gentlemen's agreement, gentleman's agreement noun

1. a personal understanding or arrangement based on honour and not legally binding