ipse dixit

Borrowed from Latin ipse dīxit (“he himself said it”), calque of Ancient Greek αὐτὸς ἔφα (autòs épha).
Originally used by the followers of Pythagoreanism, who claimed this or that proposition to be uttered by Pythagoras himself. Extended during the Middle Ages to the statements of Aristotle, and more famously used in such contexts.

• (UK) IPA^(key): /ˌɪpseɪ ˈdɪksɪt/, /ˌɪpsiː ˈdɪksɪt/
• (US) IPA^(key): /ˌɪpsi ˈdɪksɪt/

ipse dixit (plural ipse dixits)

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.
   • 1749, Henry Fielding, The History of Tom Jones, a Foundling. In Six Volumes, volume V, London: Printed by A[ndrew] Millar, […], OCLC 928184292, book i:

     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...

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

     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.

2. An authority who makes such an assertion.

• (unproved dogmatic assertion): ad verecundiam, appeal to authority, ipsedixitism

• (unproved dogmatic assertion): ad hominem, argumentum ad hominem

• “ipse dixit” in The Oxford English Dictionary, 2nd edition, Oxford: Clarendon Press, 1989, ISBN 978-0-19-861186-8.
• “ipse dixit” in Lexico, Dictionary.com; Oxford University Press.

Calque of Ancient Greek αὐτὸς ἔφα (autòs épha, “he himself said it”), originally used by Pythagoreans.

• (Classical) IPA^(key): /ˈi.pse ˈdiːk.sit/, [ˈɪ.psɛ ˈdiːk.sɪt]
• (Ecclesiastical) IPA^(key): /ˈi.pse ˈdik.sit/, [ˈiː.pse ˈdik.sit]

ipse dīxit

1. (rhetoric, chiefly Medieval Latin) Used after a statement claimed to be by Aristotle, whose absolute authority made it incontestable.