If someone is impassive or their face is impassive, they are not showing any emotion.
例句
He searched Hill's impassive face for some indication that he understood...他盯著希爾毫無表情的面孔，尋找著表明他已明白的些許跡象。As the foreman of the jury announced the verdict, Miss Allan remained impassive.陪審團主席宣布裁決時,艾倫小姐一直面無表情。
英漢詞典釋義
adv.
無感情地；無表情地；無動于衷地
Pym listened impassively until the woman had finished.皮姆一直無動于衷地聽著，直到那個女人說完。