The opposite of fat is thin. Some alternative words are slim, slender, skinny, lean, wiry, petite, and lanky. In general, thin, slim, and slender are more positive, whereas skinny is often used as a criticism or negative point.
胖的反义词就是thin（瘦）啦。其他的一些表达包括slim、slender、skinny、lean、wiry、petite和lanky。一般说来，thin、slim和slender是更具褒义的，skinny则是有一点批评的意味或者贬低的含义。

The words lean and wiry mean that the person is thin AND muscular. The word petite is only used for women, and means that the woman is short AND thin. Finally, the word lanky means tall and thin.
Lean和wiry意味着一个人不仅很苗条而且有肌肉。Petite则是用来描述瘦小的女性的。最后，又高又瘦的女性则用lanky来表达。