play possum

(idiomatic, dated) to dissemble or to feign ignorance; to disguise or conceal something in order to deceive.

Example:   Thinking fast, we played possum hoping the bear wouldn't bother us.
  The soldier played possum, fooling the sniper.
  To keep the focus away from his client, the lawyer basically played possum during the entire complex trial, and his tactic paid off with an acquittal.
  When we used to get home late at night, I would play possum so my daddy would carry me inside and put me in bed.
1833, Asa Greene, A Yankee Among the Nullifiers: An Auto-biography, p32
  Though, as it afterwards turned out, the Yankee had money enough about him, and was merely playing the ’possum all the while.
1840, Edgar Allen Poe, The Business Man, [1]
  Never imposing upon any one myself, I suffered no one to play the possum with me.
1858, James Russell Lowell, in a letter to O.W. Holmes, collected in The Complete Writings of... p31
  You have been holding-in all this while — possumus omnes, we all play the ’possum...
1881, Alexander Lovett Stimson, History of the Express Business, p354
  As none came with the coach from Deadwood, I suppose the amount of funds was insignificant. You can't tell, though, for the stage company is liable to play possum sometimes.