A choirboy is a young boy who sings in a church choir. In many churches, especially in Britain, boys have sung the high parts for hundreds of years, often in a special uniform, standing in rows at the front. The word is ordinary and a little old-fashioned; today many choirs include girls, and people may say choir member or chorister instead. Though actually, people also stretch choirboy. If they say someone looks like a choirboy, they mean he looks sweet and innocent. And if they say “he’s no choirboy,” they mean the opposite: he is not as good or as innocent as he seems. That second use is informal and a bit teasing.