From the notion of lying on one’s back, spine-bash is used in Australian English as:
– a verb meaning to rest or spend time in an aimless, idle way;
– a noun denoting an instance or period of resting.

This word originated in the slang of the Australian Army during the Second World War.

The earliest occurrences that I have found are, as spine-bashing:

1-: From “a rough glossary of Australian Army slang terms” appended to This War Is Evolving New Army Slang, by George H. Johnston, published in The Argus (Melbourne, Victoria) of Saturday 15^th November 1941:

SPINE BASHING: Having a rest; loafing.

2-: From a letter written by one H. R. Tomlinson, “a former Innisfail journalist, and now with the RAAF at a northern operational base”, published in The Evening Advocate (Innisfail, Queensland) of Monday 17^th August 1942:

“We had just finished lunch when, like a bolt from the blue, because we were not expecting anything, we were given the standby. I had sewn several buttons on a clean shirt, and was undecided whether to have half an hour in the sun, or enjoy half an hour of ‘spine bashing.’”

3-: From a report on the bomber crews stationed at a northern base, by M. Pratt, published in The Sydney Morning Herald (Sydney, New South Wales) of Friday 18^th September 1942:

Their main forms of relaxation are confined to “spine bashing”—the most popular form of spending leisure moments in the north, which is lying on your bunk reading or dozing—shooting the wild geese, pigs, and other game that abound in the area, or swimming in any of the warm, scrub-fringed pools.

As a noun, spine-bash occurs in the following from Khaki Sidelights, by ‘Trevdee’, of the Australian Imperial Force (AIF), published in Smith’s Weekly (Sydney, New South Wales) of Saturday 14^th November 1942:

In the army you are not encouraged to run around finding out why things are happening, or not happening—unless you’re a cuckoo Major or a troppo ¹ Corporal. You just sit, or lie down for a good old “spine-bash,” and await results.

¹ troppo: mentally disturbed from exposure to a tropical climate; hence: crazy, mad.

C. B. Calderwood played on the word spine-bash in the final sentence of Private Nought, published in The Sun (Sydney, New South Wales) of Saturday 21^st August 1943:

I’m doing a bit of spine bashing in my tent. Why shouldn’t I? The drongo ² does it every day.
When I say drongo, I mean the regimental sergeant-major. He’s a Joe Blake ³, he is. The only difference being that you can kill a snake, but you can’t kill the RSM—at least, that is, not in a standing camp.
Since I joined the Army two years ago, I’ve learnt a lot of answers; in fact, I think I know ALL the answers. That’s why I can spine-bash when other sports are out on the parade ground.
They gave me a nickname when I’d been in for a while. They called me “Private Nought,” because I do naught and have done naught for months.
[…]
[…] I lost my mate, Butch. He was trying to see how fast a Jeep could travel, when the crazy thing tipped on him. He did in his skull and a leg, and the MO told me that only a miracle will get him back into the Army.
Well, some blokes have all the luck. But you never know. Maybe if I bash my spine enough they’ll let me get out, too?

² drongo: a stupid or incompetent person.
³ Joe Blake (rhyming slang): a snake.