"to plead the Fifth" or "plead the 5th" is a term originating in the United States, and means that a criminal suspect asserts the right not to testify against himself in a criminal prosecution. The right against self-incrimination is given to Americans by the Fifth Amendment to the United States Constitution, and that is why we say "to plead the Fifth." "to plead ignorance" means to try to excuse oneself by stating that you weren't aware of the facts and circumstances of a particular situation. In the law, ignorance of the law is not am excuse, but ignorance of the facts may be an excuse. So if you rob a bank, but don't know there is a law against bank robbery, you will still go to jail. But if someone steals property, and sells it to you, you will not be guilty of receiving stolen property if you were ignorant of the fact the property was stolen. But if you knew it was stolen, you will go to jail. "to plead ignorance" is often used in non-legal contexts. Suppose you are invited to a party, but no one told you it's a birthday party. So you arrive without a present. When you find out it's a birthday party, you could say, "Sorry I didn't bring you a birthday present. I plead ignorance. I didn't know it's your birthday."