Wiktionary
unstiffen
vb. To remove the stiffness from; to relax; to make pliant.
Usage examples of "unstiffen".
After twenty paces he switched the box to his other hand and flailed the now-free one to unstiffen it.
Henry felt his legs unstiffen, and he somersaulted over onto the floor, lying face up, quite a bit the worse for having been uniehandled.
Instead Laurence stood up and tried to unstiffen his neck surreptitiously by small movements to either side, without any undignified stretching.