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1.
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.
by dr_binks November 27, 2005