Sissy is an American English affectionate nickname for a sister, following the same family-role naming logic as Buddy (for a male companion) or Poppa (for a patriarch figure). On a female dog, it implies the pet occupies a sister-like position in the household: either the second female animal, or simply the one the family thinks of as the girlish companion.
The Family-Role Naming Pattern
Sissy fits in the same category as Buddy, Junior, and Poppa: names that describe a family relationship rather than an identity. These names are common at lower registry ranks because they're personal and household-specific. The name makes complete sense to the family that chose it and requires explanation for everyone else.
The Actress Connection
Sissy Spacek, born Mary Elizabeth Spacek and nicknamed Sissy by her older brother, is one of American cinema's most acclaimed actresses, known for Carrie (1976) and Coal Miner's Daughter (1980). The nickname became her professional name. Some owners choosing Sissy for a female dog are nodding to that specific actress, though many are simply using the family-affection term.
The Counter-Reading: Dated Undertones
Sissy as a standalone name carries some generational dating. It reads most naturally on animals in households with Southern American or older Midwestern naming sensibilities. Younger owners may find it slightly old-fashioned. That quality is also precisely why some owners find it charming.