What does "a bit of a tartar" mean?

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!

"Characteristic of the Tartars" is basically correct. The OED describes them as:

2. fig. Tartar-like; rough and violent, savage

and

3. fig.

a. A person supposed to resemble a Tartar in disposition; a rough and violent or irritable and intractable person.

Another possible meaning, based on personal experience, is for tartar to mean stupid or mentally ill, presumably a play on retard. This comes from my schooldays, was probably local, and I haven't heard it used that way in decades.

A Tartar in this context is someone who is domineering, bossy, overbearing. It's a Britishism and quite old.

Technically and traditionally a Tartar (or Tatar) was one of the Mongolian hordes who overran Asia under Genghis Khan (although this may not be demographically correct).