"gain weight" 是增加体重的常用短语。
来源："gain" 是英文里的 "增加" 意思，"weight" 是 "体重" 意思，所以 "gain weight" 就是 "增加体重" 的意思。
英文例句及其中文翻译：
I've been trying to gain weight for months but I just can't seem to do it.
我已经试着增加体重好几个月了，但是我就是做不到。
He's been eating more in an effort to gain weight.
他一直在多吃东西以增加体重。
She's been doing weightpfting to gain weight.
她一直在做重量训练来增加体重。