We call the finger joints knuckles, but the word used to mean any joint in the body, including the elbows and knees.
To “knuckle under” is left over from those days and refers to bending your knees or bowing, signaling submission.
Kowtow is Chinese and means “to kneel and press your forehead to the ground,” which was expected in the presence of the emperor or anyone else you feared.
We say that a subordinate person is “kowtowing” to another when they are “knuckling under” to their wishes.
