1
general::
Phrase(s): six feet under
Fig. dead and buried. • Fred died and is six feet under. • They put him six feet under two days after he died.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Dead and buried, as in No, you can't read my diary? not until I'm six feet under. Although this expression alludes to what has long been the traditional depth of a grave, that is, approximately the same as the length of the coffin, it dates only from the mid-1900s.
American Heritage Idioms