heres a good link
https://en.wikipedia.org/wiki/Sissy
Now keep in mind that this has also evolved thru times and there is also Female sissys shush as myself
So in conclusion id say Sissy is what you belive it is. Its no real "strict guidelines" on what a sissy is per say as its constantly evolving
But in short Sissy is a HYPER girly girl (either a man or a woman) often highly submissive (in said roleplay ) and its usely the males that are more into the humiliation part rather then wimen sissys that likes the submissive & Girly girl part.
Sissy came from the Forced fem/Pinnaforism /petticoating
NOT BDSM and was from the start designed for males who still are the majority of Sissys that this way safely could take short trips in too the traditional (ie think back 1950 and pryor ) female gender role and as the " forced " aspect is there take away the shame otherwise would occur in many males as it is indeed a compleatly emasculating for the male (think of it as Cis wimen when they learn to be wimen ) & in basic terms its same as mother daughter in her little girl. And has since also evolved to include both Cis (born wimen ) wimen ,Transgender females, and so on. And it has also developed into further under branches under sissys shush as Sissy /Maid / Secretary /Slave / Indeed also AB/ LG /LG (the difference between
Sissy AB /LG / /Little is that Sissy little is still meant as a emasculation and humiliation under force = Forced fem. Whereas ord AB and so on is from the true desire of actually be treated and regarded as a Little.