This phrase comes from one aspect of behaviour exhibited by the Virginia opossum. Virginia opossums are well-known for their habit of `playing possum', or feigning death, when danger threatens.

　　Their passive defence strategy is not uncommon among small animals, but it is particularly well developed in the Virginia opossum. The animal curls up on the ground and stays absolutely motionless. The behaviour is a nervous reaction, comparable to fainting, and is brought about by harmless, semi-paralysing substances that are released into the bloodstream as a response to high levels of stress.

　　These substances cause the opossum's muscles to contract, and maintain the apparent `rigor mortis' until the danger has passed and the stress level drops back to normal. Though the reaction may last only half a minute, in some cases the animals remain motionless for as long as six hours.

　　At other times, the Virginia opossum is very active. It is an excellent climber, with a muscular, prehensile tail. The tail acts as a fifth limb, providing extra grip and stability as the opossum clambers among the branches in search of birds, small mammals, eggs and fruit.