"somersault" is how I'd spell it - I gather it's supposed to be from the French "sobresault" - with the "sobre" being a variation on the Latin "super". So it's about jumping over. In this case, jumping over yourself.

Doesn't seem that probable to me either. "Super" to "sobre" sounds fine - but "sobre" to "somer" sounds rather strained. You'd have expected it to turn up in English as "sobersalt" or something like that.