take 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.
Mate: I take the fifth.
Taking the fifth
Pleading the fifth after being tested positive for use of five different drugs
D'javarro: Dude, I heard Will was taking the fifth at Ryan's party last night.
You: No way! Must have been cray.
You: No way! Must have been cray.