He was completely starstruck when he met his idol and couldn't even speak.

starstruck 是由 star（星星）和 struck（strike 的过去分词，表示"被击中"）组成的复合形容词。字面意思是"被星星击中的"，引申为"见到明星时激动得不知所措的"状态，形容见到偶像时那种既兴奋又紧张的感觉。