To stiffen means to become firm, rigid, or less flexible. When you leave bread out overnight, it stiffens and becomes hard. When you're startled by a loud noise, your whole body might stiffen as your muscles tense up. A starched shirt collar stiffens the fabric so it holds its shape.

You can also stiffen something deliberately. Bakers add gelatin to stiffen whipped cream so it doesn't collapse. Engineers stiffen bridges with steel beams to keep them from swaying. When you stiffen your resolve, you strengthen your determination to keep going even when something gets difficult.

The word often describes reactions to fear, anger, or surprise. If someone stiffens when you mention a topic, their body language tells you they're uncomfortable. A cat might stiffen before pouncing, muscles coiled and ready. Notice how stiffening can be physical (your cold fingers stiffen in winter) or emotional (your pride might stiffen your refusal to apologize, even when you know you should).