ipse dixit

plural ipse dixits

1. countablerhetoricuncountableA 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, Henry Fielding, “Of the Serious in Writing; and for What Purpose It Is Introduced”, in The History of Tom Jones, a Foundling, volume II, London: A[ndrew] Millar, […], →OCLC, book V, page 113

   1 more example

   Mr. Lincoln has not character enough for integrity and truth, merely on his own ipse dixit, to arraign President Buchanan, President Pierce, and nine Judges of the Supreme Court, not one of whom would be complimented by being put on an equality with him. There is an unpardonable presumption in a man putting himself up before thousands of people, and pretending that his ipse dixit, without proof, without fact, and without truth, is enough to bring down and destroy the purest and best of living men.

   1858 August 21, Stephen A. Douglas, parliamentary debates, Ottawa, Illinois

2. countableuncountableAn authority who makes such an assertion.

   Synonyms: ad verecundiam, appeal to authority, ipsedixitism