A written statement that someone makes after promising officially to tell the truth. An affidavit can be used as proof in a law court.Word History and Origins​​ Origin of affidavit1. An affidavit is a written statement which you swear is true and which may be used as evidence in a court of law. Both copies will be required in court. An affidavit is a written statement that you swear is true and that may be used as evidence in a court of law. What does the noun affidavit mean? There is one meaning in OED's entry for the noun affidavit.