"Ipsedixitism: dogmatic assertion or assertiveness" (Merriam-Webster).
"Ipse dixit: an arbitrary and unsupported assertion" (Collins).
"Ipsedixitism: ... the practice of dogmatic assertion" (The Century Dictionary and Cyclopedia: The Century dictionary, 1889).
The charge applies when one makes an assertion without giving supporting reasons.
The word 'ipsedixitism' was introduced into English by Jeremy Bentham. He used the word and several of its cognates in his Deontology; or the Science of Morality, a manuscript that was edited by his literary executor, Sir John Bowring, and published posthumously in 1834.
In an essay that he contributed to that edition, Bowring traced the expression to Cicero:
The appellative of ipse-dixitism is not a new one; it comes down to us from an antique and high authority, —it is the principle recognised (so Cicero informs us) by the disciples of Pythagoras. Ipse (he, the master, Pythagoras), ipse dixit, —he has said it; the master has said that it is so; therefore, say the disciples of the illustrious sage, therefore so it is.(Bowring, "History of the Greatest-Happiness Principle," [pp. 287-331, at pp. 322-323], in Jeremy Bentham, Deontology; or, The Science of Morality: in which the Harmony and Co-Incidence of Duty and Self-Interest, Virtue and Felicity, Prudence and Benevolence, are explained and exemplified. Vol. I. 'Arranged and edited by John Bowring.' London: Longman, Rees, Orme, Browne, Green, and Longman, 1834)
Bowring's aetiology resembles the one given in Bentham's "Article on Utilitarianism: Long Version," according to which:
This appellative is not a new one invented for the present purpose, but an old one borrowed from an antique and consequently high authority. It is the principle pursued, so Cicero informs us, by the disciples of Pythagoras. 'Ipse' (referring to Pythagoras) 'ipsedixit': 'He has said the matter is so and so, therefore', said a disciple of the illustrious sage, 'so it is'. ("Article on Utilitarianism: Long Version," in The Collected Works of Jeremy Bentham, Vol. 5, Deontology together with A Table of the Springs of Action and Article on Utilitarianism, ed. Amnon Goldworth [Oxford: Clarendon Press, 1983], p. 305, n. 43)
It is difficult to determine the author of this last remark. Is it Bentham or Bowring? The editor of the quoted volume, Amnon Goldworth, writes that Bowring wrote sections of Bentham's posthumous works (and did not note when he had done so), adding that since parts of the original manuscript no longer exist, it is impossible sometimes to determine whether a given passage was penned by Bentham or by Bowring. Even if it was written by Bowring, Goldworth says that Bowring had discussed these works with Bentham many times and took himself, when he added material, to be adding only what Bentham had said.
Both of the above quotations mention Cicero's criticism of the Pythagoreans, which appears in paragraph 10, Book I, of Cicero's De Natura Deorum (The Nature of the Gods). Cicero wrote as follows:
Those who seek my personal views on each issue are being unnecessarily inquisitive, for when we engage in argument we must look to the weight of reason rather than authority. Indeed, students who are keen to learn often find the authority of those who claim to be teachers to be an obstacle, for they cease to apply their own judgement and regard as definitive the solution offered by the mentor of whom they approve. I myself tend to disapprove of the alleged practice of the Pythagoreans: the story goes that if they were maintaining some position in argument, and were asked why, they would reply: 'The master said so', the master being Pythagoras. Prior judgement exercised such sway that authority prevailed even when unsupported by reason. (Cicero: The Nature of the Gods, ed. and trans. Peter G. Walsh, [Oxford University Press, 1997; electronic: 2016], Book I, paragraph 10; DOI: 10.1093/actrade/9780199540068.book.1)
According to Cicero, a Pythagorean was prone to appeals to authority, resting his/her claims on their assertion by someone whose word provides insufficient support. This error of reasoning is not the same one as making assertions without giving supporting reasons. After all, the Pythagoreans did offer a reason for their claims ('Pythagoras said it!') even if not a very good one. So, the error of ipsedixitism, as defined at the beginning of this post, is not the error that Cicero charged the Pythagoreans with committing.