An Etymological Dictionary of Astronomy and Astrophysics

Any of various long-tailed rodents resembling mice but larger, especially one of the genus Rattus (TheFreeDictionary.com).

Etymology (EN): From M.E. rat, rotte, from O.E. ræt, of unknown origin.

Etymology (PE): Gerz, from (Lori, Laki) gerza “rat, big mouse,”
Gilaki (Lâhijân, Gâleš, Bandar-Anzali, etc.)
gerze “rat; mouse,” variant (Mâzandarân) gal “mouse,” of unknown origin.