Recently, a woman I know who lives in England (OK, she's my fiancee, if you want to know), wrote to me and described someone she knows as "a bit of a tartar". Now in context it seemed like a friendly jibe, but it I wasn't familiar with this particular construction, as an American.
Yes, yes, I could ask her what she means, but I wanted to ask it here and contribute to the knowledge base!
I found one dictionary definition in which it said that as an adjective it meant "of, relating to, or characteristic of the Tatars". Which of course explains everything. Not.
Edited to Add: Since this was posted, my fiancée has become my wife, whoo hoo!