A darn is a repair or fix for a hole in clothing, especially a tear.
You might use "darn" when talking about mending a favorite sweater that got snagged. Maybe someone gave it to you with a hole in it and said, "Good luck trying to darn this!" It's true, sewing up those holes can be tricky, but people have been darning their clothes for centuries. Some folks even use special yarns or threads that are just right for the job. If you're not much of a sewer yourself, you might take it to someone who specializes in repairing damaged clothes - they could darn it for you.
(euphemistic) Damn.
