fin

1) flap-like external organ concerned with locomotion in fishes. See dorsal, adipose, caudal, anal, pelvic (ventral), and pectoral fins

2) an arm or hand (slang). See also tip the fin

3) in America a $5 bill, from the Yiddish finnif, a ₤5 note in England in the nineteenth century. From the German funf or five

4) the verb for the process known as finning