take (or plead) the fifth (in the USA) exercise the right of refusing to answer questions in order to avoid incriminating yourself.
☞ The reference in this phrase is to Article V of the ten original amendments (1791) to the Constitution of the United States, which states that 'no person…shall be compelled in any criminal case to be a witness against himself'.
2013CNN transcripts We're still awaiting Lois Lerner's testimony before a House Oversight Committee. She's the IRS official in charge of the unit that oversaw tax exempt status. She is expected to plead the fifth.