His proud smile vanished after the teacher's harsh comment deflated him.

deflated 是动词 deflate 的过去式，de- 是前缀表示"去除、减少"，flate 来自拉丁词根 flare 表示"吹气"。deflate 字面意思是"把气放掉"，引申为"使泄气、使沮丧"。 在句中表示老师的批评让他像被放了气的气球一样，从骄傲状态变得沮丧。