A phrase implying that one is dead.

Presumably, this term originated with the necessity to bury the dead in such a way as to prevent the casket from rising to ground level easily in areas with easily saturated soil.

Generally speaking, in dirt that does not have liquid saturation problems, good burial rule of thumb averages around:
single depth= four and one-half feet or deeper
double depth= six feet or deeper

Soil, when saturated with water, cas easily act as a liquid and objects with low density tend to float in the right liquid environment. A good example of this is in the low country of Louisiana and Mississippi, USA, where in many places it is necessary (and even regulated) that a grave either have a heavy weight slab above it, the grave must be 8 (or more) feet deep, and even in some cases, the interment must be above ground.