A choirboy is a boy member of a choir, also known as a treble. As a derisive slang term, it refers to a do-gooder or someone who is morally upright, in the same sense that "Boy Scout" refers to someone who is considered honorable or conscientious.

What is a choir boy slang?

a boy who sings in a choir, especially a church choir. Slang. a person who is notably honest, moral, or innocent.

What does no choirboy mean?

2 : an innocent or virtuous man : altar boy sense 2 They're not thugs, but they're not choirboys either …— Phil Taylor.