spine n.

"large nail," mid-14c., perhaps from or related to a Scandinavian word, such as Old Norse spik "splinter," Middle Swedish spijk "nail," from Proto-Germanic *spikaz (source also of Middle Dutch spicher, Dutch spijker "nail," Old English spicing "large nail," Old English spaca, Old High German speihha "spoke"), from PIE root *spei- "sharp point" (source also of Latin spica "ear of corn," spina "thorn, prickle, backbone," and perhaps pinna "pin" (see pin (n.)); Greek spilas "rock, cliff;" Lettish spile "wooden fork;" Lithuanian speigliai "thorns," spitna "tongue of a buckle," Old English spitu "spit").

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.

Old English finn "fin," from Proto-Germanic *finno (source also of Middle Low German vinne, Dutch vin), perhaps from Latin pinna "feather, wing" (see pin (n.)); or, less likely, from Latin spina "thorn, spine" (see spine).

U.S. underworld slang sense of "$5 bill" is 1925, from Yiddish finif "five," from German fünf (from PIE root *penkwe- "five") and thus unrelated. The same word had been used in England in 1868 to mean "five pound note" (earlier finnip, 1839).