E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Knuckle Under (To).
To kneel for pardon. Knuckle here means the knee, and we still say a knuckle of veal or mutton, meaning the thin end of the leg near the joint. Dr. Ogilvie tells us there was an old custom of striking the under side of a table with the knuckles when defeated in an argument; and Dr. Johnson, following Bailey, says the same thing.