The Latin phrase "Ipse Dixit" means literally "he himself has said it", referring to an arbitrary and unsupported assertion meaning "the only proof we have is that somebody said it"! Lawyers use the phrase to describe dogmatic statements, unsupported by facts, which are made by "expert" witnesses. This is to remind the Judge or Jury that just because someone says it doesn't mean it is true, even if comes from an expert!

See the article Ipse Dixit Theology.