1530, from M.Fr. sombresault, from O.Prov. sobresaut, from sobre "over" (from L. supra "over") + saut "a jump," from L. saltus, from the root of salire "to leap" (see salient). Sometimes further corrupted to somerset, etc.
http://www.etymonline.com/index.php?term=somer sault
Etymology: Middle French 'sombresaut' leap, ultimately from Latin 'super' over + 'saltus' leap, from 'salire' to jump.
http://www.m-w.com/cgi-bin/dictionary?book=Dic tionary&va=somersault
Obsolete French 'sombresault', variant of 'sobresault', from Old Proven�al 'sobresaut' : sobre-, above (from Latin suprā; see uper in Indo-European roots) + saut, leap (from Latin saltus, from past participle of salīre, to leap
http://dictionary.reference.com/search?q=somer sault&r=66