jus incognitum

from Bouvier's Law Dictionary, Revised 6th Ed (1856)
JUS INCOGNITUM. An unknown law. This term is applied by the civilians to 
obsolete laws, which, as Bacon truly observes, are unjust, for the law to be 
just must give warning before it strikes. Bac. Aphor. 8, s. 1: Bowy. Mod. 
Civ. Law, 33. But until it has become obsolete no custom can prevail against 
it. Vide Obsolete. 
    

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