After years of struggling, he finally decided to throw in the towel and quit his job.

throw 是动词，意思是"扔、抛"。 in 是介词，表示"在...里面"或"进入某种状态"。 the 是定冠词，表示特指某个事物。 towel 是名词，意思是"毛巾"。 throw in the towel 是一个习语，源自拳击比赛，当拳击手认输时会扔毛巾表示放弃，后来引申为"认输、放弃"的意思。