Things which have had a certain interpretation [whose interpretation has been settled, as by common opinion] are not to be altered.

Explanation

(English-origin maxim.) Once an authoritative interpretation of a legal term or provision has been settled by common usage or judicial decision, it should not be disturbed — stability of meaning is essential to the rule of law.

Where this applies

• A word in a statute has been interpreted the same way by courts for 150 years, and a new litigant tries to argue it means something entirely different — the court refuses to disturb the settled meaning.
• The phrase "due process" has a long-established legal meaning, and a legislature tries to redefine it in a new statute to mean something narrower — courts reject the redefinition.
• A commercial term like "FOB" has been understood the same way in trade for decades, and one party to a shipping contract tries to give it a novel interpretation to avoid liability.