Sentence:  Eustace <WSD>gaped</WSD> at him in amazement. When his urbanity dropped away from him, as now, he had an innocence of expression which was almost infantile. It was as if the world had never touched him at all. 
Meaning of the word marked with <WSD> tags:  To open the mouth wide, especially involuntarily, as in a yawn, anger, or surprise.