1827 of animals (1805 of plants), from spine + -less. Meaning "lacking moral force" is from 1885. Related: Spinelessly; spinelessness.
Entries linking to spineless
spine n.
c. 1400, "backbone," later "thornlike part" (early 15c.), from Old French espine "thorn, prickle; backbone, spine" (12c., Modern French épine), from Latin spina "backbone," originally "thorn, prickle" (figuratively, in plural, "difficulties, perplexities"), from PIE *spe-ina-, from root *spei- "sharp point" (see spike (n.1)). Meaning "the back of a book" is first attested 1922.
-less
word-forming element meaning "lacking, cannot be, does not," from Old English -leas, from leas "free (from), devoid (of), false, feigned," from Proto-Germanic *lausaz (cognates: Dutch -loos, German -los "-less," Old Norse lauss "loose, free, vacant, dissolute," Middle Dutch los, German los "loose, free," Gothic laus "empty, vain"), from PIE root *leu- "to loosen, divide, cut apart." Related to loose and lease.