If your grasp of physics is vague and you've got a test coming up, it's time to hit the books. When something is vague, it’s unclear, murky, and hard to understand.
Vague comes from the Latin vagus, which means wandering or rambling. Think of a vagabond, someone who wanders around the world with only a vague idea of where he's going. There are a few big, impressive words for vague, including ambiguous, nebulous, and tenebrous.