E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Cap (the verb).
I cap to that, i.e. assent to it. The allusion is to a custom observed in France amongst the judges in deliberation. Those who assent to the opinion stated by any of the bench signify it by lifting their toque from their heads.
To cap. To excel.
Well, that caps the globe.C. Brontë: Jans Eyre.