E. Cobham Brewer 1810–1897. 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.
1
To cap. To excel.
2
“Well, that caps the globe.”—C. Brontë: Jans Eyre.
