1. To become rigid and difficult to bend or fold. This material is very pliable when wet, but it stiffens up as soon as it gets dry. Using starch in the wash will cause the fabric to stiffen up.

2. To cause something to become rigid and difficult to bend or fold. In this usage, a noun or pronoun can be used between "stiffen" and "up." I'd like to find a way to stiffen up the brim of my hat. You'll need to use a special solution to stiffen the fabric up.

3. Of one's body part, to become difficult or painful to move or bend. Your muscles are going to stiffen up if you don't keep stretching and moving around. My wounded leg stiffened up while I was sleeping, making it hard to walk in the morning.

4. To cause one's body or body part to become difficult or painful to move or bend. In this usage, a noun or pronoun can be used between "stiffen" and "up." The spy stiffened up his neck as soon as he felt the cord being slipped around it. Make sure not to stiffen your arm up while the tattoo is being applied.

5. To become resistant to free or easy movement. After years of disuse, the hinges on these old gates have completely stiffened up. Make sure you keep the bicycle chain well lubricated so it doesn't stiffen up in this wet weather.

6. To cause something to become more resistant to free or easy movement. You should try stiffening the shocks up on your motorcycle. Exposure to moisture must have stiffened up the operating mechanism.

7. To become abruptly cold, reserved, or unreceptive, as in reaction to some appeal, suggestion, idea, etc. The boss always stiffens up whenever you try to ask for a budget increase. Jack stiffened up at the mere mention of Janet's name.