They were worried he might have another nasty surprise up his sleeve.

have 在这里是动词，表示"拥有、持有"。 up 在这里是介词，表示"在...上方"或"在...内部"的方位关系。 sleeve 的意思是"袖子"，指衣服覆盖手臂的部分。 have something up one's sleeve 是一个习语，字面意思是"袖子里藏着某物"，引申为"暗藏玄机、另有打算"的意思。