He was blindsided by someone rushing out from the alley and knocked to the ground.

blindsided 是动词 blindside 的过去分词形式。blindside 由 blind（看不见的）和 side（侧面）组成，字面意思是"从看不见的侧面"，引申为"出其不意地攻击"或"使措手不及"。这里表示"被突然袭击"或"被意外击中"。