道貌岸然
Look as if butter would not melt in one's mouth.

eg：The girl looks as if butter would not melt in her mouth when she sits next to her teacher.

中英俚语俗语来源：续仙传：“李延情景恬檐，道貌秀异。”

同义词：一本正经Have a serious look.
反义词：贼眉鼠眼 Have the brand of a villain in one's looks.