mos pro lege

From mōs (“custom, habit”) + the preposition prō (“for”) + lēge, the ablative of lēx (“law”). Meaning "custom for law".

• (Classical Latin) IPA^(key): /moːs proː ˈleː.ɡe/, [moːs̠ proː ˈɫ̪eːɡɛ]
• (modern Italianate Ecclesiastical) IPA^(key): /mos pro ˈle.d͡ʒe/, [mɔs prɔː ˈlɛːd͡ʒe]

mōs prō lēge

1. custom for law; meaning that the usage that is customary has the force of being a law