From Longman Dictionary of Contemporary Englishon your backon your backinformal not polite if someone achieves something on their back, they achieve it by having sex with someone → back
Examples from the Corpus
on your back• He took Deianira on his back and in midstream insulted her.• For a lew moments I lay spread-eagled on my back, unable to change position.• Thousands of people have been practicing step aerobics, but over-enthusiasm could leave them flat on their backs.• It appears to be a board-room decision and not his so let's not get on his back too quick.• She lay on her back and laughed at the sky.• Forget Michelangelo, lying on his back high above the Sistine Chapel floor to paint his famous frescoes.• He lay grinning up at me on the water, spreadeagled on his back with his arms splayed. 