Definition of "asseverate"

Whereas Your Maiesty hath received true information of great sums of Money endeavoured to be borrowed of Your City of London, by directions proceeding from both Houses of Parliament […] (I Your Maiesties faithfull Subiect) doe conceive, and dare asseverate, that it is intended upon no other pretence then for the Honour and defence of Your Maiesty,

1642, Sir Richard Gurney, 1st Baronet, The Lord Maior of Londons Letter to the King at Yorke, June 22‎, London

I will myself asseverate and bind it by an Oath, that the Muff thou bearest in thy Hand belonged unto Madam Sophia;

1749, Henry Fielding, chapter 7, in The History of Tom Jones, a Foundling, volumes (please specify |volume=I to VI), London: A[ndrew] Millar, […], book 10, page 261

“And I presume you can also asseverate to his worship, that no man is better qualified than I am to bear testimony in this case, seeing that I was by you, and near you, constantly during the whole occurrence?”

1817 December 31 (indicated as 1818), [Walter Scott], chapter IX, in Rob Roy. […], volumes (please specify |volume=I to III), Edinburgh: […] James Ballantyne and Co. for Archibald Constable and Co. […]; London: Longman, Hurst, Rees, Orme, and Brown, pages 195-196

The more I laughed at such theories, the more these stubborn friends asseverated them; adding that even without the heritage of legend the recent reports were too clear, consistent, detailed, and sanely prosaic in manner of telling, to be completely ignored.

1931, H. P. Lovecraft, chapter 1, in The Whisperer in Darkness

A time would come, Mr. Secombe-Hughes mysteriously asseverated, when all would be well and she would get her money.

1971, Iris Murdoch, An Accidental Man‎, New York: Viking, page 31