ipse dixit

noun2

1. •
   (rhetoric) A dogmatic and unproved proposition or dictum that is accepted solely on the authority of someone who is known to have asserted it.

   • To, avoid, therefore, all Imputation of laying down a Rule for Posterity, founded only on the Authority of ipse dixit; for which, to say the Truth, we have not the profoundest Veneration; […] — 1749
2. •
   An authority who makes such an assertion.