Pinky Swear/Promise

I was wondering how to say this in Polish and if this something commonly done/said.

Thanks! Dziekuje!

there is no such thing in Polish culture as pinky swear and therefore there are no words that describe it

Often, one cannot "translate" an expression or idiom, because the culture reflected in or by the other language has no known equivalent. Conversely, an example in Polish might be "Psakrew, bydło!", literally, "Dog's blood, cattle!". This sounds plain laughable in English. Therefore in this case, an entirely different phrase must be concocted out of the original. In the above instance, "Dammit!" might suffice, whilst a similar expression "Cholera!" (literally "Cholera!", just as in English) might be rendered in much the same way. If one were to cry out "Cholera!" in English, it might cause a panic rather than indicate a mere release of anger and frustration:-)

I've never heard such thing.

Besides that the OP meant an oath form not a curse word.

You have never heard "psia krew"?? I don't know what "pinky swear" mean though

I have heard psia krew, but I hadn't heard "psa krew" :)
I've also never heard anyone saying "bydło" as a kind of curse. People use that term to describe certain behaviour of group of people but not as a curse word.

google it marcpol

zetigrek but 'psiakrew, co za bydło' is plausible (dog's blood, what a cattle)- a lovely swear btw :)