If you darn something knitted or made of cloth, you mend a hole in it by sewing stitches across the hole and then weaving stitches in and out of them.

A darn is a part of a piece of clothing that has been darned.

People sometimes use darn or darned to emphasize what they are saying, often when they are annoyed.

You can say darn it to show that you are very annoyed about something.

You can say I'll be darned to show that you are very surprised about something.