If something is stale, it's no longer fresh. Ever bite into a piece of bread that's been left out a little too long? Chances are it's stale, or dry and hard to eat. Try making some croutons out of it.
The word stale comes from the Old French estaler meaning "to halt," which is what happens to your jaw when you try and bite down on a piece of stale bread — it just can't chew through it. But stale isn't a word that's used only when talking about food. When an idea is stale, it's old or boring. And when a horse stales, it's not old and crusty, it's urinating. Step aside.