Boy Scout

Someone who plays by the book, regardless of the situation, especially to stand out in contrast to others who would prefer to do something differently. Sometimes he helps his friends stay out of trouble, and sometimes he’s just a buzzkill who whines a lot. It’s almost always a derogatory term, and has nothing to do with the actual Boyscouts of America (though the image conveyed is that of a goody goody little Boy Scout using a diagram to tie knots and helping old ladies cross the street).