Brewer's: 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.