Distinguishes from 开衫 (kāishān, cardigan — the open-front kind) and 卫衣 (wèiyī, hoodie/sweatshirt). 套头衫 is specifically defined by its construction: no buttons or zip, you pull it on over your head.
memory
套 (tào, to slip on / pull over — same 套 as in 套上手套 'put on gloves') + 头 (tóu, head) + 衫 (shān, shirt). Literally 'a shirt you pull over your head'.
