To cap the climax

to surpass everything, as in excellence or in absurdity.

See also: Climax

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in classic literature ?
And, to cap the climax of their base ingratitude and fiendish barbarity, my grandmother, who was now very old, having outlived my old master and all his children, having seen the beginning and end of all of them, and her present owners finding she was of but little value, her frame already racked with the pains of old age, and complete helplessness fast stealing over her once active limbs, they took her to the woods, built her a little hut, put up a little mud-chimney, and then made her welcome to the privilege of support- ing herself there in perfect loneliness; thus virtually turning her out to die!
Nothing but that was wanted to cap the climax! That's what he's been bothering his wits about these two years past!"
And then, to cap the climax, the floor boss would come rushing up with a rifle and begin blazing away!