That was a phrase used to describe the Israelites who wandered around in the desert for 40 years.
The term stiff necked is one drawing its origin from the horse world at that time.
Some people might have experienced this when trying to turn a horse in a direction not desired by the horse. Horses also have the instinct to raise their heads and use the muscle on the bottom of their necks when they are approached by another horse that they feel is threatening to them. When they are frightened, their neck stiffens with the raised head and their legs often become immobile. They adopt the stiff bottom neck from the time they are born- the underside of the neck is their nursing muscle. The muscle in the bottom of the neck gets lots of action during halter tag. When the bottom of their necks are used their backs and loins are not being built up. Their front legs take on more concussion. The shoulders become stiff as well. It suits the one-on- one strength combat with another equine, but it is counterproductive for riding and working with humans.
How often we are like the horse. Unwilling to change direction, feeling threatened or frightened and trying to accomplish things in our own power from the time we are born. Not developing the “muscles” that give us true strength.