Many of the etymologies of Romanian counties are Romanian interpretations of Slavonic names (e.g.: Gorj and Dolj), as the administration documents in the Middle Ages Romanian Principalities (Wallachia and Moldavia) were written in this language.
Possibly from Rossy vel Jargo, Jurcova or Zorio.
(see external link).
Possibly named after Giurgiu as the Genoese in the 14th century named it after San Giorgio, the patron of their city.