Possible interpretations:
* A fictional creature: It could be a name given to a fantastical animal, perhaps in a story or game.
* A descriptive term: It might be used to describe a real animal with a particularly agile tail, like a squirrel or a cat.
* A brand name: It's possible that "nimbletail" is a trademarked name for a product or service.
To understand the meaning, you need more context. For example, where did you encounter the term "nimbletail"? Was it in a book, a movie, or a conversation?
Let me know more about where you saw the word, and I might be able to give you a better answer.