If you stiffen, you stop moving and stand or sit with muscles that are suddenly tense, for example because you feel afraid or angry.

If your muscles or joints stiffen, or if something stiffens them, they become difficult to bend or move.

If attitudes or behaviour stiffen, or if something stiffens them, they become stronger or more severe, and less likely to be changed.

If something such as cloth is stiffened, it is made firm so that it does not bend easily.