taking the Fifth

Definition from Nolo’s Plain-English Law Dictionary

A popular phrase that refers to a witness's refusal to testify on the ground that the testimony might incriminate the witness in a crime. The principle is based on the Fifth Amendment to the U.S. Constitution, which provides that "No person . . . shall be compelled to be a witness against himself," and is applied to state courts by the Fourteenth Amendment. (See also: self-incrimination)

Definition provided by Nolo’s Plain-English Law Dictionary.