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Phrase(s): Ignorance (of the law) is no excuse (for breaking it).
Prov. Even if you do not know that something is against the law, you can still be punished for doing it. (An ancient legal principle.) • Police officer: I’m giving you a speeding ticket. Motorist: But I didn’t know I was exceeding the speed limit! Police officer: Ignorance of the law is no excuse for breaking it. • Terry protested that he didn’t know it was illegal to break the windows of an abandoned building, but the judge informed him that ignorance of the law was no excuse.
McGrawhill's American Idioms And Phrasal Verbs