Cissy

From UnknowableWiki

Cissy is a nickname for Narcissa Malfoy. It is first revealed when Bellatrix tries to coax Narcissa out of going to Snape's home when the latter wants Snape's help after her son Draco has been ordered to kill Dumbledore (HBP2). Bellatrix refers to Narcissa as Cissy once more, when she is worried about the sword of Gryffindor and Voldemort's wrath (DH23).

Cissy seems to be a name confined to the Black household; aside from Bella, the only one to refer to her as such is Kreacher, who uses it when he is explaining the story of Regulus to Harry (DH10).