goody two-shoes

A prudish, self-righteous individual, a goody-goody. For example, Phyllis was a real goody two-shoes, tattling on her friends to the teacher. This expression alludes to the main character of a nursery tale, The History of Goody Two-Shoes (1765), who was so pleased when receiving a second shoe that she kept saying “Two shoes.” The goody in the story is short for goodwife but means “goody-goody” in the idiom.