1. To refuse to testify against oneself in court, in accordance with the rights guaranteed by the Fifth Amendment to the United States Constitution. The alleged kingpin of the east coast drug cartel simply pled the Fifth after every question the prosecution put to him.

2. By extension, to refuse to answer a question or provide information, especially if doing so may incriminate or embarrass oneself. Just plead the Fifth if your mom asks where you've been all night! A: "So, I hear things got pretty messy at the bar last night." B: "Yeah, I'm going to have to plead the Fifth Amendment on that one!"