/ ˈstɪfn; NAmE / verb
1.
stiffen (sth) (with sth) to make yourself or part of your body firm, straight and still, especially because you are angry or frightened :
[ v ]
She stiffened with fear.
[ vn ]
I stiffened my back and faced him.
2.
stiffen (up) | stiffen sth ( of part of the body ) to become, or to make sth become, difficult to bend or move :
[ v ]
My muscles had stiffened up after the climb.
[ vn ]
stiffened muscles
3.
to make an attitude or idea stronger or more powerful; to become stronger
SYN strengthen :
[ vn ]
The threat of punishment has only stiffened their resolve (= made them even more determined to do sth) .
[also v ]
4.
[ vn ] stiffen sth (with sth) to make sth, such as cloth, firm and unable to bend