To lie about or misrepresent the truth about some event, person, or thing. In common usage, it usually refers to perjury (telling a lie while under oath in a court of law) or to the Ninth Commandment of the Old Testament, from which the phrase is taken: "Thou shalt not bear false witness against thy neighbor." Even if it is a small lie and seems harmless, if you bear false witness under oath, you may end up going to jail yourself.