a girlyboy is a young male, usually adolescent, who looks and/or acts effeminate. He is often assumed to be gay and usually has an interest in things that would interest the average female his age - fashion, pop music, romantic/sexual attraction to other boys, etc. Either due to social isolation from the "alpha males" his age or by personal choice, his close circle of friends are usually female. In contrast to a "ladyboy" (transvestite or transsexual), a girlyboy does not necessarily have a desire to dress as or become a "woman", instead he is naturally feminine in his tastes, appearance and/or behavior. He is often unintentionally mistaken for a girl by those who first meet him and may be secretly considered attractive by straight/masculine males his own age who would ordinarily be attracted only to females.
Cindy's 14-year-old brother is a such a girlyboy, I thought he was her younger, hotter sister before she introduced him.