nope, Playing possum describes playing dead, which is what Opossums do when they approach a severe fight-or-flight situation which causes them to go into a catatonic state that is designed by nature to make them appear dead to predators, who then might decide to avoid them (unless they are the kind of animal that likes carrion)
"the fix was in",..........would that count :dunno: People must have been "fixing" things long before boxing.
I don't think that's where that comes from :-/ I reckon 'on the back foot' is probably one 'take it on the chin'