Roseusboy is defined as a masculine gender that has many parts of femininity incorporated into the overall gender identity, but still has plenty of masculine qualities as well.[1]
Roseusboy is closely related to gnc male identity, and can be used as a synonym for that, but it can also be a gender in its own right. A gender that, while fundamentally male/masculine, has so many gnc aspects to it, on such a deep level, that they cause the gender to no longer quite be experienced as simply that anymore.
It can also refer to any gender that combines masculine and feminine elements into a whole that is ultimately masculine, with significant feminine aspects.
It can also be interpreted as having a focus on the concept of the "parts", ie, a gender that is made up of multiple genders or fragments of gender, which when combined add up to something that is ultimately masculine, despite many of the parts being more feminine, or even bits of female gender, when viewed on their own.
It was coined by tumblr user genderpotion, who also created the flag, and later expanded on in discord conversations between people identifying with it.