Green’s Dictionary of Slang

an unsupported statement.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1869		Will-o-the-Wisp 1 147/2: Mr. Disraeli, M.P. gets up and condemns a bill being brought in on the strength o' the ipse dixy (that'll puzzle folks down in Puddledust) of the Irish Attorney.
1870		Christian Misc. Mar. 119/1: But he always sifted an opinion or a sentiment, avowing that he would never be bound by any man's Ipse dixy.
1874		Macmillan’s Mag. 29 512/1: The sheriffs officer and the attorney's clerk brought their learned technicalities out of Cursitor Street and the Old Bailey, so that now ipsal dixal stands for ipse dixit, and a davy is an affidavit.
1895		Truth (Sydney) 17 Feb. 3/4: Then we sticks a blooming bank, / All in Joseph’s ipse-dixey.