"spine, backbone," c. 1300, from Anglo-French achine, Old French eschine (11c., Modern French échine), a word of uncertain origin, perhaps from Germanic (compare Old English scinu "shinbone;" see shin).
"the primitive backbone," 1848, coined in English by English anatomist Sir Richard Owen from chord (n.2) + Greek nōton "back," which is perhaps from the same PIE source as Latin natis "buttock," which is the source of Italian and Spanish nalga, Old French nache "buttock, butt." Related: Notochordal.
1814, "a cross," from Latin crux "cross," a word of uncertain origin. Sometimes said to be cognate with Irish cruach "heap, hill," Gaulish *krouka "summit," Old Norse hryggr "backbone," Old English hrycg "back." But de Vaan is suspicious:
The Celtic and Gm. forms are often reconstructed as *kr(e)u-k-, but we find vacillating vocalism within Gm.; also, the meanings 'backbone' and 'heap' are not necessarily connected. Even if the words in *kruk- from Latin and Italo-Celtic belong together, the root structure does not look PIE (and a root enlargement k is unknown), and might be interpreted as a non-IE substratum word borrowed into Italo-Celtic. But Latin may also just have borrowed the word from a contemporary language.
The figurative use for "a central difficulty" (1718) is older in English than the literal sense; perhaps it is from Latin crux interpretum "a point in a text that is impossible to interpret," the literal meaning of which is something like "crossroads of interpreters." But Century Dictionary ascribes it to "the cross as an instrument of torture; hence anything that puzzles or vexes in a high degree ...." Extended sense of "central point" is attested by 1888.
The English word also might be influenced by and partly a borrowing of Latin spica (see spike (n.2)), from the same root. Slang meaning "needle" is from 1923. Meaning "pointed stud in athletic shoes" is from 1832. Electrical sense of "pulse of short duration" is from 1935.
Middle English rigge, from 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"). OED says "of uncertain relationship;" Pokorny, Boutkan, and Watkins have it from PIE *kreuk-, extended form of root *sker- (2) "to turn, bend."
The original "back" sense, predominant in Middle English, seems to have become archaic 17c. Also in Old English, "the top or crest of anything," especially when long and narrow, based on resemblance to the projecting part of the back of a quadruped, the "ridge" of the backbone. Probably also in late Old English "a long elevation of land, a long, narrow range of hills," implied in place-names. From late 14c. of the highest part of the roof of a building, also the strip of ground thrown up between two plowed furrows. The spelling with -dg- is from late 15c.
Ridge-runner, somewhat derisive term for "Southern Appalachian person, hillbilly," especially an upland white farmer of the Ozarks region, is recorded by 1917 (it later came into use in other regions). Also "person who wanders from place to place," often with a suggestion of illicit intent (1930).