I'm combing the list of Three-Letter Words, but it's hard to say that many are cool. They are either commonplace or obscure. This word is obscure, but it suits a purpose I didn't even know existed, but now that I know the word, I plan on using it.
Why it's useful: V, U, and G are unhelpful letters that don't go with other letters very well. Best to get rid of them quickly.
Why it's cool: Whenever you see a rock that has an opening with crystals inside, you are looking into a vug. Here are many examples of what I'm talking about. But you can use the word to describe any opening in a rock. Verdict: cool.