Its usage is explicitly called out in Charles Dickens' Hard Times: For These Times (first published in 1854), and remained in common usage at least in the United States until the 1950s and 1960s, although it has since fallen out of favor (coinciding with the rise in its usage as a pejorative). Its usage as a diminutive for Cecilia dates back to at least the late 19th century. Sissy is also a term of endearment used as a diminutive for the female given name Cecilia.
Research published in 2015 suggests that the terms are asymmetrical in their power to stigmatize: sissy is almost always pejorative and conveys greater severity, while tomboy rarely causes as much concern but also elicits pressure to conform to social expectations. Sissy is, approximately, the male converse of tomboy (a girl with masculine traits or interests), but carries more strongly negative connotations. A man might also be considered a sissy for being interested in traditionally feminine hobbies or employment (e.g., being fond of fashion), displaying effeminate behavior (e.g., using hair products, hydrating products, or displaying limp wrists), being unathletic, or being homosexual.
Generally, sissy implies a lack of courage, strength, athleticism, coordination, testosterone, male libido, and stoic calm, all of which have traditionally been associated with masculinity and considered important to the male role in Western society. Sissy (derived from sister), also sissy baby, sissy boy, sissy man, sissy pants, etc., is a pejorative term for a boy or man who is not traditionally masculine, and shows possible signs of fragility.