The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with six feet under
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.