Long underwear, also called long johns or thermal underwear, is a style of two-piece underwear with long legs and long sleeves that is normally worn during cold weather. 

长款内衣，也叫long johns或thermal underwear，是一种上下两件的带有长裤腿和长袖的内衣，通常在寒冷季节穿。

 

所以long john就是秋衣秋裤的意思，如果要单指秋裤，可以说成long john pants/bottoms。