Old English hrycg "back of a man or beast," probably reinforced by Old Norse hryggr "back, ridge," from Proto-Germanic *hruggin (source also of Old Frisian hregg, Old Saxon hruggi, Dutch rug, Old High German hrukki, German Rücken "the back"), of uncertain origin; perhaps from PIE *kreuk-, extended form of root *sker- (2) "to turn, bend."
Also in Old English, "the top or crest of anything," especially when long and narrow. The connecting notion is of the "ridge" of the backbone. Spelling with -dg- is from late 15c. Ridge-runner "Southern Appalachian person" first recorded 1917.