Sentence:  He was seeing , somehow , the face of a young boy , the boy Simms Purdew must once have been , a boy with sorrel hair , and blue eyes dancing with gaiety , and the boy mouth <WSD>grinning</WSD> trustfully among the freckles . 
Meaning of the word marked with <WSD> tags:  a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement