undershirt
noun [ C ] US uk/ˈʌn.də.ʃɜːt/ us/ˈʌn.dɚ.ʃɝːt/ (UK vest)
（无袖保暖）内衣;（贴身穿的）背心
a type of underwear that covers the upper part of the body, worn for extra warmth
a cotton/wool undershirt 纯棉／羊毛背心
She always wore a long-sleeved thermal undershirt in the winter. 她冬天总是穿一件长袖保暖内衣。
Compare: singlet