The chocolate cake looks great, but I have to watch my weight.
四级词汇讲解:
watch作为动词原意是“看”，而这里“留意，监督”。因此，watch my weight含义为“注意我的体重”,言外之意就是不能吃这块巧克力蛋糕，否则会变胖的。
英语四级考点归纳:
watch的用法还有:
Watch the birdie!瞧这边!/照相啦!(美国俚语)
Watch for fires!小心火灾!
Watch yourself.留点神。
Watch closely. I'll make turns.盯紧点，我要转弯了。
Watch your step. 小心脚下。(相当于Mind your step. )
Watch your mouth.说话留点神。(用于警告别人，相当于Watch your tongue/language，或Becareful of what you say.)
From:https://www.zgghmh.com/CET-4/20003527.asp