Source : Webster's Revised Unabridged Dictionary (1913)

Possum \Pos"sum\, n. [Shortened from opossum.] (Zo["o]l.)
   An opossum. [Colloq. U. S.]

   {To play possum}, {To act possum}, to feign ignorance,
      indifference or inattention, with the intent to deceive;
      to dissemble; -- in allusion to the habit of the opossum,
      which feigns death when attacked or alarmed.

Source : WordNet®

possum
     n 1: nocturnal arboreal marsupial having a naked prehensile tail
          found from southern North America to northern South
          America [syn: {opossum}]
     2: small furry Australian arboreal marsupials having long
        usually prehensile tails [syn: {phalanger}, {opossum}]