He likes to wear a sweater-vest over his shirt, which keeps him warm and stylish.

sweater-vest 是复合名词，由 sweater（毛衣）和 vest（背心）组成。sweater 指针织的毛衣，vest 指无袖的上衣。sweater-vest 合起来指"针织背心"，是一种无袖的针织上衣，通常穿在衬衫外面。