Definition of Rhea sister in US English:

Rhea sister

(also Ria sister, Rea sister)

noun

humorous
  • In plural Diseases ending in -rhoea or -rhea, as diarrhoea, gonorrhoea, pyorrhoea, etc., personified.

Origin

1930s. From -rhea + sister, with -rhea treated humorously as a surname.

Pronunciation

Rhea sister

/ˈriːə ˌsɪstə/