Pretend to be dead or asleep, as in Max always plays possum when it's time to clean up his room. This expression alludes to the fact that the opossum falls into an apparent coma when caught, giving the appearance of death. [1820s]

To pretend to be dead, a trick used by opossums to defend themselves from predators: “Everyone thought the old con man had died, but it turned out he was just playing possum.” By extension, it also means “to pretend to be asleep, or to lie low”: “Come on, Harry, open your door. You can't play possum forever.”