Amendment V (the Fifth Amendment) of the United States Constitution, which is part of the Bill of Rights, states:
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
To plead the Fifth is to invoke the Fifth Amendment's protection that no person "shall be compelled in any criminal case to be a witness against himself" -- that is, to refuse to testify on the grounds that one's testimony might be taken (or mistaken) as incriminating oneself. The expression is also used as a metaphor for refusal to comment on a suspicious or "incriminating" circumstance. It has become well known in Western popular culture, thanks particularly to American crime dramas on television -- so much so that police and judges in nations which recognize no such right have found themselves faced with suspects who wish to plead the Fifth!. compare with the right to silence
Contrary to popular belief, the Fifth Amendment is not limited to the right to refuse to testify. It also provides for a grand jury for major crimes, forbids double jeopardy, requires the application of due process, and requires payment for the taking of private property. Through the Fourteenth Amendment, the United States Supreme Court has applied all of the Fifth Amendment's guarantees, except the grand jury requirement, to state governments as well.