The phrase "a relentless back" is correct and usable in written English.
It can be used in contexts where you are describing someone who is persistently supportive or unyielding, often in a physical or metaphorical sense.
Example: "During the match, he played with a relentless back, never giving up on any point."
Alternatives: "an unyielding support" or "a tireless defender."
Unstable hothead Anakin (the millstone around the series' neck) improbably gets a Padawan, or apprentice -- the carefully calculated-to-be-cute Ahsoka Tano -- and their relentless back and forth brainless sniping, often amid combat, is parsecs from engaging.
It was a welcome relief from the relentless, back-breaking work of the unit.
Every season it is generally Phillips that trails far behind Sotheby's and Christie's both in the amount of art it is able to sell and in the calendar of a relentless week of back-to-back auctions in New York every May and November.
The relentless back-and-forth usually lasts for five or more hours a day and sometimes all day on weekends.
6 Terminator (1984 /T2: Judgment day (1991) Dir: James Cameron Robots from 2029 send a relentless cyborg (Arnold Schwarzenegger) back to 1980s Los Angeles to assassinate the mother of a future human rebel.
But a relentless Lancers team battled back, surging back in front and ending the third with a 50-46 cushion.
In the post, written shortly after the change in prime ministership last September, Lyle Shelton wrote that "Abbott, a social conservative, had been effective in holding back a relentless campaign to change the definition of marriage in recent years".
It has been a relentless spell that stretches back to the US Open which began on 12 June.
What is left is a relentless pressure to cut back on the single most expensive cost centre at media companies: The content creation engine, a.k.a., our newsroom.
Therefore the last off ramp -- the expression is a favorite of Secretary of State Colin L. Powell -- is the one that leads to a relentless United Nations inspection program, backed by a credible threat of force, that persuades President Saddam Hussein to surrender everything that could be construed as illicit weapons or the banned tools for making them.
