Darn is a mild exclamation people use when they're frustrated, disappointed, or annoyed but don't want to use harsh language. You might say “Darn, I forgot my homework!” or “Darn it, I struck out again!” It expresses the same feeling as stronger curse words but in a gentler way that is more acceptable in polite company.

The word can also mean to repair a hole in fabric by weaving thread back and forth across it. When you darn a sock, you stitch new threads over the hole to close it up. Before clothing became inexpensive and easy to replace, people regularly darned their worn-out socks and sweaters instead of throwing them away. Your great-grandmother probably knew how to darn, using a wooden mushroom-shaped tool to stretch the fabric while she worked.

As an adjective, darn works like “very” to add emphasis: “That's a darn good sandwich!” Someone might also say “I'll be darned” when they're genuinely surprised by something unexpected.