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: Webster's Revised Unabridged Dictionary (1913)