To "take the 5th" or "plead the 5th" means to claim protection under the fifth amendment to the U.S. Constitution. This amendment says in part that no one "shall be compelled, in any criminal case, to be a witness against himself." If a person believes that his testimony might tend to provide incriminating evidence against himself, he can invoke the fifth amendment and he will not be required to answer.
Interviewer: Why did you leave your last job?
Interviewee: They accused me of stealing four computers
Interviewer: Did they make you confess?
Interviewee: I took the fifth
Refuse to answer on the grounds that one may incriminate oneself, as in He took the Fifth on so many of the prosecutor's questions that we're sure he's guilty. This idiom refers to the Fifth Amendment to the U.S. Constitution, which states that no person shall be compelled to be a witness against himself or herself. Mid-1900s
Me: What was you just doing with my wife!?
Mate: I take the fifth.