This ad struck me as funny for the close association in spelling to Fink.
The word origin of Fink comes from the German language, similar to the word for Finch. It had a use on college campuses for someone not in a fraternity, possibly because they were "wild" or uncaged. The german slang term for dirty or low is Schmutzfink. What ever the origin I can imagine that it helped the sales of this likely durable overalls.