While I am familiar with most of the terms batted about here, one term commonly used in reference to one's kilt eludes me. I know this may sound dumb, but what is meant by a "tank"?

Oh, and is there a glossary of sorts I can consult with such questions in the future? I tried searching the site for an answer, but the word is used so frequently that I had more hits that I could peruse for an answer.