Definition of knuckling:
(n) :
Any of the joints between the bones of the fingers.
(n) :
(by extension) A mechanical joint.
(n) :
A contrivance, usually of brass or iron, and furnished with points, worn to protect the hand, to add force to a blow, and to disfigure the person struck; a knuckle duster.
(v) :
(transitive, slang) To strike or punch.
(v) :
(intransitive) To bend the fingers.
(v) :
(intransitive, figurative) To yield.