"Fink" has disappeared from our vocabulary. I think it was popular in the 1960s. Before that, I think it was used by union organizers. A fink was an employee who spied on the union and reported to management. Then to "fink on" someone was to spread any information that would get them in trouble. Later "fink" became a popular but vague term of abuse.

I think people liked the word because it sounded funny. Any word with a K in it is funny. Koalas, kangaroos, and kinkajous are funny. Bats, deer, and wolves are not funny.