Sissy (derived from sister; also sissy baby, sissy boy, etc.) is a pejorative term for a boy or man who violates or does not meet the traditional male gender role. Generally, sissy implies a lack of courage and stoicism, which are thought important to the male role. A man might also be considered a sissy for being interested in traditional feminine hobbies or employment (e.g., being fond of fashion), displaying effeminate behavior (e.g., saying "mua mua" before hanging up the phone or using creams), being unathletic, or being homosexual.
Sissy is, approximately, the male converse of tomboy (a girl with masculine traits or interests), but has none of the latter's positive connotations. Even amongst gay men, behavior thought of as sissy or camp produces mixed reactions. Some men reclaim the term for themselves.
The term sissyphobia denotes a negative cultural reaction against "sissy boys" thought prevalent in 1974. Sissyphobia has more recently been used in some queer studies; other authors in this latter area have proposed effeminiphobia as an alternative term.
Read more about Sissy: Term of Affection Toward Women, In Gender and Queer Studies, In Sexual Subcultures