A little more history about this funky little word. It’s a noun pronounced fngk it is amazing to see how words shift about as society changes. It first appeared in print as a colloquial in a Rudyard Kipling poem 1890 poem Wee Willie Winkie (1890) I don't fink I'll ever want to kiss big girls. H.G. Wells used it this way too in You Can't be Too Careful (1941) That boy is still as pure as the driven snow. (I don't fink. I saw his face.)

Originally most expert sources say it comes from German where fink was "a frivolous or dissolute person," originally a variant of finch, a perfectly nice bird as in bullfinch or goldfinch, which also gave it the sense of a person who is an "informer."

By the early 1900's the word took an interesting twist. Archie Green writes in his labor lore in folklore book called Torching the Fink Books and Other Essays On Vernacular Culture tracing the background though the practice of Sailor's Union of the Pacific in 1934 where employees were required to carry booklets called "fink books." Burning them in protest the seamen objected to having to record the expectations to carry these records intended for identification and performance records of seamen.

It's an American slang and pejorative term by today's standards whose origins remain largely unknown. It appears first in 1903 in a general sense as an unpleasant or contemptible person. In 1914 Jackson & Hellyer Vocabulary for Criminal Slang lists a fink as ' an unreliable confederate or incompetent sympathizer. It's interesting to note the use of the word confederate and wonder if this reflects a relationship to the American Civil War. Eleven years later the word was being compared and contrasted in American Speech I: `Dick' and `bull' and `John Law' have become established as names for the police, while `fink'and `stool' and `fly-dick' denote the plain-clothes men. Makes me think of gangsters and Tommy guns .......rat-ta-tat-tat!

As the New Year arrived in 1926 another publication (America Mercury) talks about what most experts believe to be the origin of this little four letter word and how it gained its notoriety. Fink blew a fuse during the famous Homestead Strike of 1892 as an odious expression. According to one version fink was originally Pink, a contraction of Pinkerton, and referred to the army of strikebreakers recruited by the detective agency. During the labor struggles of the late nineteenth and early twentieth century there was a strike in 1892 at the Carnegie Steel Company in Homestead, Pennsylvania. Employed by the bosses were spies and informers, these fink were deplored even more so than the scabs who crossed picket lines.

By tracing fink as a rhyming variation on the name of the Pinkerton Detective Agency, this popular and sometimes discounted theory relates their agents would frequently infiltrate unions as spies for management and played a pivotal role in the ruthless suppression of the Homestead strike:

For almost five months in 1892, the Homestead lodges of the Amalgamated Association of Iron and Steel Workers and the Carnegie Steel Company, Limited clashed over contract negotiations in what has become known as The Homestead Strike.....

(T)he notorious Henry Clay Frick …known for his ruthless anti-union policy and as negotiations were still taking place he ordered the construction of a solid board fence topped with barbed wire around mill property. The workers dubbed the newly fortified mill "Fort Frick."

Workers expressed their discontent by hanging Frick and superintendent J.A. Potter in effigy on mill property and turning the hose on the men sent by Potter to cut the effigies down. With this event as an excuse, the company began to shut down the works on June 28.

Union and non-union workers joined forces under the leadership of Hugh O'Donnell and kept guard around the steel works to prevent any black sheep, or scabs, from entering. Frick meanwhile, had already made arrangements with Pinkerton's National Detective Agency of New York for the arrival of 300 strikebreaking detectives, commonly known as "Pinkertons."

Once it was clear that the detectives planned on docking at and entering mill property, workers tore through a company fence to stop them.... who fired the first shot remains a mystery, the detectives opened fire on the crowd and wounded several workers. The workers hid behind ramparts of steel, pig iron and scrap iron and returned fire while the women and children retreated out of range. The battle lasted from 4 a.m. on July 6 until 5 p.m., with workers finally agreeing to the surrender of the Pinkertons. Three Pinkertons and seven workers died and many more were wounded in the fight.

Named after the famous Scotsman Allan Pinkerton, he established the Pinkerton National Detective Agency in 1850. The were so successful in protecting railroad property strike breaking that it many hoboes, railroad hijackers, and union members alike also hate the "Pinks" or "Pinkies."

After a six day siege the Pennsylvania governor called in the National Guard to restore order but in the end, steel baron Andrew Carnegie who had fled the states, and Frick succeeded in preventing any organizing of the mills for the next forty years. "Fink" in this sense became "traitor" and was such a powerful slur that it fairly quickly passed into general usage meaning "a contemptible, disloyal person." Sometimes it is used synonymously with strike breaker today too. Some general words related to fink are rat, nark, sneak, stoolie and, stool pigeon. The public's intense dislike for finks has since found expression in a newer and even more derogatory name for this enduring breed, the ratfink, an idiom coined in the 1960s. Wonders, as well as finks, will never cease!

Sources:

Etymology:
http://www.geocities.com/etymonline/

fink:
http://www.wlu.edu/~hblackme/oed/fink.html

The Homestead Strike 1892 :
http://www.bgsu.edu/departments/