be agape and tongue-tied; be at a loss for words; be stumped; stare(at sb) in mute amazement (/with the mouth open/with a fallen jaw)
❍ 末了朝贾正一问，问得贾正真有点张嘴结舌，吱吾了半天，……(冯志《敌后武工队》207) …and his question to Jia Zheng made Jia gape for some time before he could reply.
❍ 地主婆和狗管家被问的～，无话可说。(石文驹《战地红樱》129) The landlord’s concubine and his flunkey both stood there speechless.
❍ 方兆元～。(张骏祥、赵拓《白求恩大夫》59)Fang stares at him.
❍ 若飞同志有力的论证，驳得伪法官们～，期期艾艾地讲不下去了.只好宣布退庭。(杨植霖《王若飞在狱中》29) Wang’s powerful arguments tied the tongues of the Kuomintang judges and reduced them to incoherent stammering. They had no alternative but to declare the court dismissed.
张口结舌zhānɡ kǒu jié shé
结舌：舌头不能转动。张着嘴说不出话来。形容理屈或紧张害怕而说不出话来。be at a loss for words, stunned, gape in astonishment, see a wolf, with open mouth