I have heard of it in British English - I am sure I saw it mentioned in a play. The Collins-Pons dictionary claims it to be Scots English or American English. Rather than fingers being held up, the promise was made by 2 people interlocking 4th fingers with one another.

It does seem to be a children's thing, even though pinky/pinkie to mean your 4th finger (thumb excluded!) seems to be more adult used - e.g. drinking with your pinkie/pinky out (not nearly as rude as it sounds, but a social faux pas never the less!)

As to a translation to it in German... Well I'm flummoxed there I can only offer etw. beschwören - it may be a good case for cultural transplantation - unless of course the type of sworn oath is of importance.

Thanks a million for your prompt reply, Mike!

Appreciate it!

yes, this is used quite freuqently by young kids among their friends. I would say if someone in the 5-10 yr old age group uses it, it would literally be swearing. In other words, kids this young would be taking their little "pinky swear" as if it were an oath of death. (Pinky swears are often accompanied by interlocking the pinky finger, as someone already said). You will hear older kids and even teenagers use it to, but often in a more light and sometimes even sarcastic way in the sense of "really?" "pinky swear?" etc, but older kids would not actually do the "motion" and interlock their fingers.

"Yeah, I'll make sure to tell him. I promise"
"Pinky swear?" (in a half joking/half serious I-dont-really-believe-you-will-do-it way)
"yeah."

my 2 cents :)

one more thing:

Adults would use this phrase, but mostly when talking with small kids. Parents, for example, would use "do you pinky swear" or similar expressions with "to pinky swear" to ask the kid to promise/swear to do something or to make sure a kid is telling the truth.

Thanks, ma! That was very enlightening!