By the 1960s fink was generally used by kids as a safe non-swear word. It was in Rocky and Bullwinkle in 1961.
We all would have gotten that.
But by then it was mostly a kid’s term. I don’t think adults bothered to use it because they had real swear words they could get away with.
If they did use it, they used it more or less jokingly. And if you haven’t seen Rat Pfink a BooBoo - and who has, nowadays? - run, don’t walk, to whatever outlet you use for weird old movies. It’s an experience like nothing else. Not a good experience, but an Experience.
Fink was standard US slang for a person (usually a guy) with poor character. My dad & his friends used the term all the time to describe one or another jerk at work. I & my friends used it for other kids who were a PITA. At the time I thought it had some connontation of a suck-up, teacher’s pet, tattle-tale. But that wasn’t the primary thing it meant.