take the fifth
take the fifth {v. phr.}, {informal}
1. Taking refuge behind the Fifth Amendment of the Constitution of the United States which guarantees any witness the right not to incriminate himself while testifying at a trial.
Alger Hiss took the Fifth when asked whether he was a member of the Communist Party.
2. Not to answer any question in an informal setting.
Have you been married before? — I take the Fifth.
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Source: A Dictionary of American Idioms