From an article written by SissyBarbiePau for sissipedia dot wiki
A sissy is a non-conformant biological masculine who adopts hyper-feminine behaviors and practices (e.g., crossdressing, putting on make-up), typically assuming the submissive role to a dominant female and/or male partner(s). This can be interpreted as a previous state to a transgender or transsexual stadium.
In the past a sissy was considered only a person that had effeminate manners or was weak compared to real men. Nowadays this term is in constant evolution and change since there was new generations that now use this gender as a validation for themselves to become femme.
It can also be considered that a sissy is an intermediate kind of gender that is not classified in the Cisgender or the Transgender categories. This term can be reclaimed by Transgender people as this intermediate stage is considered a previous phase for the male to female gender reassignment.