You can check the definition of this idiom here.
a terrible show has the same name....
I don't want to be six feet under yet. I'm too young to be.
to pop off or be a goner
another good american english idiom for that would be to buy a farm, or simply, to buy it. for example: he bought it during a skirmish with the jerries. also, when somebody dies, they croak. to give up the ghost, to kick the bucket, to fall off one's perch, to bite the dust, to pop one's clogs - all of these idioms mean to die in colloquial american & canadian english. i'm sure brits, aussies, and kiwies if not use, then are at least familiar with the expressions.
six feet under - is this idiom used formal or informal ( colloquial) speech?
And another one: 'He's pushing up the daisies" (Pushing up the daisies - Idiom Definition - UsingEnglish.com ) Same idea.
But why 6 feet? I've heard it said that a scavenging animal could smell a rotting corpse in a shallower grave, but I'm not sure about that.
Because it is the traditional depth of a human grave in the culture(s) of origin and usage of the idiom.
I learn it from a name of a TV drama