asseverate
To assert earnestly, seriously, and confidently
transitive
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,
Where it comes from
From Latin assevērāre, from assevērō (“to assert”).
Adapted from the Wiktionary entry asseverate (CC BY-SA 4.0).