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.
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 or displaying limp wrists), being unathletic, or being homosexual.
Among members of a Detroit youth gang in 1938-39, sissy was "the ultimate slur" used to tease and taunt other boys, as a rationalization for violence against rivals, and as an excuse for not observing the dictums of middle-class decorum and morality.
Thus, it appears that sissy served as a multi-functional insult, which was used not only to harass the person or group at whom it was directed, but also to reinforce gender stereotypes and gender binarism, assert men's dominance over women, disparage the weak and shy as well as the homosexual and transgender, penalize behavior that varied from gender norms, encourage aggressive behavior, and critique members of higher socioeconomic strata.
The use of the word sissy was "ubiquitous" among delinquent American youth of the 1930s; the term was used to provoke boys to join gangs, demean boys who violated group norms, force compliance with the mandates of masculinity, and justify violence (including sexual violence) against younger and weaker children.
The word sissy in its original meaning of "sister" entered American English around 1840-1850 and acquired its pejorative meaning around 1885-1890; the verb sissify appeared in 1900-1905.By the 1930s, "there was no more damning insult than to be called a sissy" and the word was widely used by American football coaches and sports writers to disparage rival teams and encourage ferocious player behavior.