Cissy is another spelling of sissy, used especially in informal British English. It is an unkind word. People have used it of a boy or man they think is weak, easily scared, or “too much like a girl” — as if being gentle were something wrong. Children used to shout it in the playground; adults who still use it often sound dated, and many people now hear it as both rude and unfair. You may meet it in older stories or in someone’s angry speech. You do not need it in your own mouth. Though actually the same letters can be a nickname from the name Cecilia, and that is a different, harmless use. The dictionary sense here is simply the “sissy” taunt, with this spelling.