They arranged the party behind his back to surprise him.

behind 是介词，意思是"在...后面"，表示位置或时间上的在后。 his 是代词 he 的所有格形式，意思是"他的"。 back 是名词，意思是"背部、后面"。 behind his back 是一个固定短语，字面意思是"在他背后"，引申为"背着他、瞒着他"的意思。