A “simple” way to explain it would be the following:
Sissy (derived from the English word Sister) is a pejorative term for a child or a man who violates or does not comply with the standard male gender role. Generally, sissy involves a lack of courage, strength, coordination, testosterone or the male libido, which are traditionally important for the male role. A man can also be considered a queer by being interested in traditionally feminine pastimes or by his employment (for example, being a fashion buff), which shows effeminate behavior (for example, the use of excess hair products or showing taste for women’s clothing), have a delicate body, or be homosexual.
Sissy is the opposite of a tomboy (a girl with masculine traits or interests), but it has a negative connotation. Even among gay men, behavior regarded as Sissy produces mixed reactions. But some men claim the term for themselves.
Sissy as a role game
In “Sissy role play” a sissy is a person (usually a man) who adopts hyper-feminine behavior and engages in “feminine” stereotyped activities (eg, housekeeping, using makeup), often in the context of BDSM.
Particularly in the BDSM community, the sissy usually assumes the role of submission to a dominant female and / or partner (s). Often, they also identify with other fetishes and sexual practices.