On his first day, the boss threw him in at the deep end by giving him a major client to handle.

threw 是动词 throw 的过去式，意思是"扔、抛"。 him 是人称代词 he 的宾格形式，指"他"。 in 是介词，表示"进入...状态"。 at 是介词，表示"在...位置"。 the 是定冠词，表示特指。 deep 是形容词，意思是"深的"。 end 是名词，意思是"末端"。 throw someone in at the deep end 是一个英语习语，字面意思是"把某人扔进深水区"，比喻"让某人直接面对困难或挑战性的情况"，类似于中文的"赶鸭子上架"或"直接面对困难"。