Fin

google

ref

Old English finn, fin, of Germanic origin; related to Dutch vin and probably ultimately to Latin pinna ‘feather, wing’.

wiktionary

ref

From Middle English fin, from Old English finn, from Proto-Germanic *finnō, *finǭ(“dorsal fin”) (compare Dutch vin, German Finne, Swedish finne, fena), from Proto-Indo-European *(s)pīn-(“backbone, dorsal fin”) (compare Old Irish ind(“end, point”), Latin pinna(“feather, wing”), Tocharian A spin(“hook”), Sanskrit स्फ्य(sphyá, “splinter, staff”).

From Yiddish פֿינף‎ (finf, “five”). Doublet of five, pimp, and finnuf.

etymonline

ref

fin (n.)

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).