Sissy

Sissy is an English diminutive and pet form of names beginning with the sound "sis," most commonly used as a nickname for Cecilia, Cecily, or Frances, though it also arose independently as an affectionate term derived from the word sister. Its use as a given name in its own right became established in the nineteenth century, particularly in the American South. The name shares its roots with other pet forms such as Cissie and Cissy, which follow the same pattern of softening longer formal names into familiar, affectionate short forms.