sweater vest       
noun [ C ] mainly US uk/ˈswet.ə ˌvest/ us/ˈswet̬.ɚ ˌvest/ (UK also tank top)
 毛衣背心
a knitted piece of clothing (= made from wool or thick cotton ) that covers the upper part of the body but not the arms, and is usually worn over other clothing
  He looks like a kindly geography teacher in his neat sweater vest, shirt and tie.  他穿着整洁的毛衣背心、衬衫和领带,看起来像一位和蔼可亲的地理老师。 
 Compare: tank top
 - More examples
 - He was wearing his usual grey trousers, a sweater vest, tie, and navy-blue blazer.
 - She looked through racks of shorts to match the polo shirts and sweater vests she had already picked out.
 - What she calls a tank top, this American calls a sweater vest.
 - Nowadays in the city's churches, you’ll see everything from jeans to miniskirts to sweater vests.