Sprezzatura is a kind of studied carelessness or seemingly effortless grace. Someone who always manages to look chic, even in their weekend clean-out-the-garage clothes, displays sprezzatura — they make it look easy to look good.
The word sprezzatura is borrowed from Italian, and it dates back to the 1500s, during the Renaissance. It was used to describe a desirable quality amongst the nobles — making difficult things appear easy and acting in a cool, nonchalant manner. Today, the word is often used in the arts, especially in fashion, where it refers to a relaxed but stylish look that seems like you didn't really bother (but you did) — e.g., a partially untucked shirt, a slightly crooked tie, a fancy dress paired with sneakers.