Did you expect quirk to be a noun meaning "a peculiarity of action or behavior"? If so, you're probably not alone; the "peculiarity" sense of the noun quirk is commonly known and has been a part of our language since the 17th century. But quirk has long worn other hats in English, too. The sense meaning "a curve, turn, or twist" has named everything from curving pen marks on paper (i.e., flourishes) to witty turns of phrase to the vagaries or twists of fate. In contemporary English, the verb quirk can be used in referring to facial expressions, especially those that involve crooked smiles or furrowed eyebrows.