A phrase meaning to give in to a superior, be it economical, physical, or otherwise.
e.g. "I don't want to have to knuckle under to Jim about that proposal. He's my boss and all, and tenured, but still.
The origin of this phrase stems from how, in pre-14th century english, all joints were known as knuckles. Thusly, when one would grovel or be down on one's knees before one's superior, one would be knuckling under to their authority.