Little boys who go against traditional male gender roles and who may have an interest in wearing women's clothing and jewelry as children.
Analogous to tomboy and similar to the word "sissy," but meant to be positive and used by people who support how children express themselves. Young children shouldn't be labeled by adults (as straight/queer/cisgender/trans) before they can self-identify.
Coined, perhaps, by author Cheryl Kiladavis in her book, "My Princess Boy".
This is my son: he's a princess boy. Maybe he'll be my daughter someday, but that's up to him. Or her.