If someone perjures themselves in a court of law, they lie, even though they have promised to tell the truth. Witnesses lied and perjured themselves. perjure yourself to tell a lie after promising to tell the truth in a court of law (parjurer, from perjurare, from jurare )
To cause to violate an oath or a vow; to cause to make oath knowingly to what is untrue; to make guilty of perjury; to forswear; to corrupt; often used reflexively; as, he perjured himself