1
general::
Seek out something or someone that turns out to be unexpectedly unpleasant or formidable, as in Now that she finally agreed to meet with you, you just might find that you've caught a Tartar. In this term, Tartar signifies a difficult or sometimes violent individual, referring to Turkic and Mongolian invaders of Asia in the Middle Ages. [Mid-1600s]
American Heritage Idioms