n. An unreliable person; someone who agrees to do something, but never follows through.
1. John called in sick to work again today. He's such a flake.
2. Mary said she would do the research for our project, but it's been a week and she hasn't done a thing. She's such a flake.
v. To decline to give self-incriminating information. Refers to the fifth amendment of the US Constitution, which states that no citizen "shall be compelled in any criminal case to be a witness against himself..."
Joe: Bob, did you ever do drugs in high school?
Bob: I'm gonna plead the fifth on that one, Joe.