What is the X in X-mas?
c.1400, "backbone," later "thornlike part" (early 15c.), from Old French espine (French épine), from Latin spina "backbone," originally "thorn, prickle," from PIE *spei- "sharp point" (cf. Latin spica "ear of corn," Old Norse spikr "nail;" see spike (n.1)). Meaning "the back of a book" is first attested 1922.
See spinal column.
Any of various short pointed projections, processes, or appendages of bone.