if you stiffen, your body suddenly becomes firm, straight, or still because you feel angry or anxious [≠ relax]:
He touched her, and she stiffened.
2 [intransitive and transitive]
to become stronger, more severe, or more determined, or to make something do this:
a campaign to stiffen rules against drunk driving
Their opposition only stiffened my resolve.
3 [intransitive] also stiffen up
to become painful and difficult to move:
His joints had stiffened.
My back had stiffened up.
to make material stiff so that it will not bend easily