A male character who is flamboyant and campy in their style and actions; applicable to men of any sexual persuasion, and of any physical build. Notably attractive to rampant Sparkles, Rainbows, Bishie Sparkles, and/or pastel colors.
The term "fabulous" for describing such flamboyant characters can be traced to a Meme involving the mannerisms of Koizumi Itsuki of Suzumiya Haruhi No Yuuutsu and the theatricality of Lelouch Vi Britannia (particularly, his Zero persona) in Code Geass. There, it is written as "FABULOUS".
An example of a work with plenty of fabulous to go along with the Manly is Araki Hirohiko's Jojo No Kimyou Na Bouken.