He shook his head when I asked him to join us.

shook 是动词 shake 的过去式，意思是"摇动、晃动"。 his 是代词 he 的所有格形式，表示"他的"。 head 是名词，意思是"头部"。 shook his head 是一个固定搭配，表示"摇头"的动作，通常用来表达不同意、拒绝或否定的意思。