A flower boy is a boy who both looks and acts like a girl. They tend to have feminine habits, such as wearing makeup, and will also be much prettier than any girl you will ever see.
Girl 1: Have you ever seen Ren from Nu'est?
Girl 2: He is such a flower boy.
Girl 1: I know right! Why can't I be as pretty as he is?
Stands for "Scum Fuck Flower Boy" attributed to the album by Tyler, The Creator, Flower Boy. It is when you are at an all time low but madly in love. Can be abbreviated to SSFB
Flower Boy in the LGBTQ community can be used to refer to any male/masculine identified individual (i.e. cis gay male, trans man) who also has a feminine side, and is soft and feminine in either personality, self-expression/style, or both. The soft boi variation of the word can also refer to someone who considers themself only partially male/masculine.
A Flower boy: growing up I was always told I was "too soft" and needed to "man up", I'm glad that I'm just a proud flower boy who isn't ashamed of that anymore."