Sissy is a derogatory name for someone, especially a man or boy, who seems weak or cowardly, or who acts in a stereotypically feminine way.
People who call others sissies aren't just insulting the so-called sissy. They're also implying that there is something weak and fearful about being a girl or a woman — it's the same as telling a boy, "You throw like a girl." It's unkind and also inaccurate. Sissy comes from sister, and its earliest use to mean "effeminate man" was in the late 1800s.