What does "opossum" mean?

Opossum: Any American marsupial of the genera Didelphys and Chironectes. The common species of the United States is Didelphys Virginiana. Note: Several related species are found in South America. The water opossum of Brazil (Chironectes variegatus), which has the hind feet, webbed, is provided with a marsupial pouch and with cheek pouches. It is called also yapock. Opossum mouse. (Zoöl.) See Flying mouse, under Flying. -- Opossum shrimp (Zoöl.), any schizopod crustacean of the genus Mysis and allied genera. See Schizopoda.