Threadbare describes fabric that's been worn so thin from use that you can see the individual threads, or even see through it in places. A threadbare blanket might have been someone's favorite for years, washed and used so many times that it's become thin and fragile, with the weave showing through clearly. An old teddy bear might have threadbare patches on its paws from years of hugs.

When cloth becomes threadbare, it's usually on the edge of developing holes.

People use threadbare more broadly to describe anything worn out from overuse. A threadbare excuse is one that's been used so many times it no longer works: “My dog ate my homework” is threadbare because teachers have heard it a thousand times. A threadbare argument has been repeated so often that it's lost its power to convince anyone. In these cases, threadbare suggests something that might have been good once but has been used up, like a joke that stops being funny after you've heard it too many times.