ipse dixit

Meanings

1. A dogmatic and unproved proposition or dictum that is accepted solely on the authority of someone who is known to have asserted it.
2. An authority who makes such an assertion.

Etymology

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.