Of American-English origin, the phrase six feet under means dead and buried (synonyms: to turn up one’s toes, to push up daisies and French manger les pissenlits par la racine, literally to eat the dandelions by the root).

Short for buried six feet under ground, this phrase alludes to the normal depth of a grave.

The complete form of the phrase appeared for example in the following from The Oregon Weekly Statesman (Salem, Oregon) of Wednesday 9^th November 1870:

NEW CHINA BILL.

Just to show how leading Democrats treat the dreaded Chinese question, we publish and call attention to the following Legislative proceedings:
Mr. Hendershott, on October 21^st, asked and obtained leave to introduce a bill—S. B. No. 97—to enable the State of Oregon to have a dead thing on Chinamen. [Laughter.]
Be it enacted by the Legislative Assembly of the State of Oregon:
Sec. 1. No Chinaman shall be allowed to die in this State, until he has paid ten dollars for a new pair of boots with which to kick the bucket.
Sec. 2. Any Chinaman dying under this act shall be buried six feet under ground.
Sec. 3. Any Chinaman who attempts to dig up another Chinaman’s bones, shall first procure a license from the Secretary of State, for which he shall pay four dollars.
Sec. 4. Any dead Chinaman who shall attempt to dig up his own bones without giving notice to the Secretary of State, shall be fined one hundred dollars.
Sec. 5. Any Chinaman who shall be born without bones for the purpose of wilfully and feloniously evading the provisions of this act, shall be fined five hundred dollars.

The Hungarian-born American magician and escape artist Harry Houdini (born Erik Weisz – 1874-1926) mentioned this particular depth, according to his obituary published in The Morning Leader (Regina, Saskatchewan, Canada) of Tuesday 2^nd November 1926:

“Out in Los Angeles eight years ago I made a bet that I could be manacled and buried alive six feet below the surface and get back to the land of the living without aid,” the magician said in recounting his experience. “The only condition I made was that the burial should be graded, first going under one foot of soil, then two, and so on. A party of us left Los Angeles at dawn and motored over the road to Santa Ana. I knew something of the geology of those parts, and I knew that surface vegetation was nothing but beard bristle on sandy soil. I was duly manacled in the graged [sic] graves, making my way out (hand and ankle free) from the shallower graves, but finding a little difficulty with the four-foot and five-foot plantings.
“Somebody urged me not to try the six-foot grave, but, as it always the way of a mystifier who makes any pretense to wisdom, I had devoted more time to practice than my friend imagined. They remanacled me, and the extra foot of soil was dug up. I was buried and the soil dumped down on me expeditiously, as stipulated.
“The shallower internments had accustomed me to the darkness and deafness of burial, yet the knowledge that I was six feet under sod—the legal requirement for corpses—gave me the first thrill of horror I had ever experienced in my career as a journeyman daredevil.
“The momentary scare—the irretrievable mistake of all daredevils—nearly cost me my life, for it caused me to waste a fraction of breath when every fraction was needed to pull through. I kept the sand loose about my body so that I could work dexterously. I did. But as I clawed and kneed the earth my strength began to fail. Then I made another mistake. I yelled. Or, at least, I attempted to, and the last remnants of my self-possession left me. Then instinct stepped in to the rescue. With my last reserve I fought through, more sand than air entering my nostrils. The sunlight came like a blinding blessing, and my friends about the grave said that, chalky pale and wildeyed as I was, I presented a perfect imitation of a dead man rising.
“The next time I’m buried it will not be alive—if I can help it.”

The earliest occurrence of six feet under in the Oxford English Dictionary (2^nd edition, 1989) is from The American Thesaurus of Slang: A Complete Reference Book of Colloquial Speech (New York, Thomas Y. Crowell company, 1942) by Lester V. Berrey and Melvin Van den Bark:

Dead and buried […] six feet under.

But the earliest instance of the phrase that I have found is from title given to the following paragraph published in The Evening Leader (Staunton, Virginia, USA) of Wednesday 17^th September 1924:

About Six Feet Under.—A wild ride in an auto ended in a flower-bed, says a news dispatch rfom [sic] Hawarden. More frequently they end under flower-beds.—Cherokee, (Iowa) Chief.

The second-earliest occurrence of the phrase that I have found is from the review of The Devil’s Cargo, a 1925 American silent drama film directed by Victor Fleming (1889-1949) and distributed by Paramount Pictures—review published in The Atlanta Constitution (Atlanta, Georgia, USA) of Sunday 1^st February 1925:

The story is one of Sacramento in 1850—a goldrush town where a crooked look meant a fight, with one of the contestants winding up “six feet under.”

I have also found an early figurative use of six feet under in the column Sport Ink-Lings, by ‘Otto’, in the Wisconsin Rapids Daily Tribune (Wisconsin Rapids, Wisconsin, USA) of Thursday 14^th July 1927:

Merrill baseball fans held a meeting Tuesday night to decide upon the future of their baseball club. The feeling displayed at this meeting indicated that the fans didn’t care much whether they had a team in the Jenny town or not, but the directors reserved their decision on whether the club will disband or not until later in the week. Popular opinion is that after the Mosinee game Sunday the Jenny club will be shrouded and put “six feet under.”