E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Knock Under (To).
Johnson says this expression arose from a custom once common of knocking under the table when any guest wished to acknowledge himself beaten in argument. Another derivation is knuckle underi.e. to knuckle or bend the knuckle or knee in proof of submission. Bellenden Kerr says it is Te nôck ander, which he interprets I am forced to yield.