It has become an idiom, and 6 feet under is practically synonymous with death. It’s a poetical euphemism, a reference to an old burial custom, but why were graves made 6 feet deep originally?
The burial rule of six feet originates in 17th century England when the Great Plague of London struck, and people were terrified of the spread of disease, so they decreed that the depth of a grave should be six feet or more to prevent the stench of Decay from escaping and so that the animals would not dig up the dead bodies.
