‘Bone of contention’ refers to an issue which remains in dispute, with the disagreeing or disputing parties unable or unwilling to resolve it. According to the Oxford English Dictionary, the idiom came into existence in the 1500s. It was coined by taking into account the theory of two dogs at a place with only one bone. What would happen in this situation? Quite obviously, the two dogs will battle over that one particular bone. So that bone will become the bone of contention between the two dogs.

 For example: The issue of Kashmir has been a bone of contention between India and Pakistan for several years now.