I wouldn't like a job like that, especially in the summer, when it's boiling hot.

boiling 是动词 boil 的现在分词形式，boil 本意是"煮沸、沸腾"。这里 boiling hot 是固定搭配，表示"极其炎热的"，用沸腾的状态来比喻高温程度。