It refers to a witness's use of the Fifth Amendment to the US Constitution (The right against self-incrimination) to avoid answering questions that could tend to incriminate them.
It doesn't mean anything. However, to "plead the fifth" means you are refusing to testify because you are invoking your 5th amendment right not to incriminate yourself. This is frequently misused on...
When a witness or defendant testifies under oath and doesn't want to answer a specific question because doing so might be incriminating, he or she might respond with "I plead the fifth" or "I take...
In the 5th Amendment to the United States Constitution it says:
"No person... shall be compelled in any criminal case to be a witness against himself..."
This means a person on trial is free to...