发胖用英语可以表达为 "gain weight" 或者 "put on weight"。这两个词都表示体重增加的意思。

来源：这两个词都是由动词 "gain" 和名词 "weight" 组成，"gain" 指的是增加或获得，而 "weight" 指的是重量。所以 "gain weight" 就是指体重增加。

例句及翻译：

I gained weight during the winter because I was eating too much. (我在冬天发胖了，因为我吃得太多了。)

My doctor told me to put on weight because I'm underweight. (我的医生告诫我要增加体重，因为我体重过轻。)