unwritten law

Translations
неписаный закон
The unwritten law of the office is to always refill the coffee pot if you take the last cup.
Неписаный закон офиса — всегда доливать кофе в кофейник, если ты взял последнюю чашку.
Definitions
An unwritten law refers to a rule or principle that is not formally codified or documented but is generally accepted and followed by a group or society. It often encompasses social norms, customs, and ethical standards that guide behavior and decision-making.