This means to 'plead the fifth amendment' (US)
The fifth amendment is;
Amendment V (the Fifth Amendment) of the United States Constitution, which is part of the Bill of Rights, is related to legal procedure. Its guarantees stem from English common law as established by Magna Carta in 1215. For instance, grand juries and the phrase "due process" both trace their origin to Magna Carta.
Here is the legal definition:
"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 offense 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."