Wiktionary
adv. (context archaic English) For certain.
Usage examples of "of a surety".
Better of a surety is a lowly peasant who serveth God, than a proud philosopher who watcheth the stars and neglecteth the knowledge of himself.
Whilst he gazed on the marvels of Westminster Hall and of old Saint Paul's he had longed that she should be near him, so that he might watch the brilliance of her eyes, and the glow of pleasure which, of a surety would have mantled in her cheeks when she was shown the beauties of the great city.
His voice was soft and dreamy, and he woke me and announced that he had seen a sign in the heavens, and knew of a surety that he was to be the next Earth King.
The lady and the two children are, no doubt, innocent tools in his hands, just as I am, and when he no longer needs them he will deliver them over to the Committee of Public Safety, who will, of a surety, condemn them to death.
We may not know of a surety till evening when, if they be not altogether gone, they will return.
And He said unto Abram, Know of a surety that thy seed shall be a stranger in a land not theirs, and they shall reduce them to servitude and shall afflict them four hundred years: but the nation whom they shall serve will I judge.
Behold of a surety thou art not able to have two Paradises, to take thy fill or delight here in this world, and to reign with Christ hereafter.
Know thou of a surety that thou oughtest to lead the life of a dying man.