Thinking about this term I found myself wondering where the first element came from. I knew the second element would be from the Latin saltire, to leap, and I think there’s a French verb sauter with the same meaning, but whence somer-? OED enlightened me. Interestingly the original word in English was sobresault which for some reason changed to somersault. Now the etymology was clear, ie supra - saltus, to leap over or above. The form sobresault, obsolete now, was still current in the 17th century as the cite in OED from one of Fletcher’s lesser-known plays, The Woman’s Prize, evidences. (I must get round to reading this one, it’s a sequel to The Taming of the Shrew, with Petruchio, now a widower, launching into a second marriage and finding that he is now the one being tamed.)
a1625 J. Fletcher Womans Prize iii. ii, in F. Beaumont & J. Fletcher Comedies & Trag. (1647) sig. Ooooo3/2, What a sober salt When the chaire fel, she fetchd, with her heels upward.
BTW I had no idea that desultory has its origins in the desultores, guys in the Circus of Ancient Rome who rode two or more horses and would leap from one to the other. One qualification: OED refers only to a Roman vaulter but the Wordsworth Dictionary of Phrase and Fable elaborates. Not the most authoritative of sources, hence the qualification, but if true it’s a great word origin.