take/plead the Fifthidiom
Definition of take/plead the Fifth
: to refuse to answer questions in a court of law because the answers might be harmful to one or might show that one has committed a crime When called to testify, he took the Fifth. —often used figuratively He took the Fifth when his wife asked him where he'd spent the night.
Note: The phrase take/plead the Fifth refers to the Fifth Amendment of the U.S. Constitution, which says that citizens of the U.S. cannot be required to give testimony that could be used against them in a court of law.