The Haniwa Look (埴輪ルック) is a Japanese slang expression popularized around 2005 relationg to female Japanese fashion.
Japanese schoolgirls who have the Haniwa Look are girls who wear their schoolgirl uniforms, specifically a Japanese schoolgirl skirt, with shorts or pants(usually sweatpants) under it. The term Haniwa refers to a type of earthen statue that was erected over the graves of powerful Japanese men in Japanese history. The silhouette of a Japanese schoolgirl in a skirt with pants under it is said to resemble the thick legged silhouette of the Haniwa statue. The term was probably not coined as a compliment in that the legs of the Haniwa statues look quite fat.
Japanese schoolgirls are generally believed to adopt the Haniwa Look (sweats under skirts) for two main reasons: