As a kid, I spent countless hours playing in my grandparents' backyard.

backyard 是复合名词，由 back（后面）和 yard（院子）组成，字面意思是"房屋后面的院子"。在美式英语中特指住宅后面用篱笆围起来的私人庭院空间，通常用于休闲活动。