finyoudaoicibaDictYouDict[fin 词源字典]
fin: [OE] Fin is a word common to the Germanic languages of northeast Europe (German has finne, Dutch vin), but its ultimate source is not clear. The likeliest candidate is Latin pinna ‘feather, wing’ (source of English pin, pinion, and pinnacle), although another suggestion is Latin spīna ‘thorn, spike’.
=> pin, pinion, pinnacle[fin etymology, fin origin, 英语词源]
fin (n.)youdaoicibaDictYouDict
Old English finn "fin," from Proto-Germanic *finno (cognates: 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 (see five) and thus unrelated. The same word had been used in England in 1868 to mean "five pound note" (earlier finnip, 1839).