A pinky promise (or "pinky swear") is a gesture in which two parties interlock little fingers in a symbolic gesture of agreement. What is the origin of this phrase?
One possibility, and probably the most straightforward one, is that it is an independent invention to describe an existing cultural practice of sealing promises by interlocking little fingers.
Another possibility, however, is that it originates from Japanese. Japanese sources frequently suggest that the English phrase "pinky promise" is a translation/adaptation of a similar Japanese concept called 指切り (yubikiri, lit. "finger-cutting") which supposedly has its origins in yakuza loyalty practices (if someone promised to do something and performed a yubikiri, and then broke their promise, their finger would be cut off).
However, as Wikipedia notes, this practice (if not the exact phrase "pinky promise") has been attested since 1860, as recorded in Bartlett's Dictionary of Americanisms. The Japanese yubikiri predates that1, so it's still possible that "pinky promise" derives from yubikiri, but I can't imagine there were many borrowings/translations from Japanese that long ago.
1 The Nihon Kokugo Daijiten, which is closest thing Japanese has to the OED, attests yubikiri in the sense of "a promise" from 1638, and in the sense of "cutting off a finger" from 1692.
Best Answer
There's a Japanese and Chinese tradition called "red thread". Wedded couples tie a red thread on their pinky finger, because they believe that it was destiny that connects them together. The red thread signifies eternal togetherness. You use your pinky finger as a promise that you are never supposed to break.
From that tradition a popular kid's rhyme:
指切りげんまん
うそ付いたら
針千本飲ます
指切った
Which means:
Cut your finger and swear
Should you lie
Swallow a thousand needles
Cut your finger
So we see there is long standing traditions that were used in Japan to denote a truly high promise to the pinky. Then it became popular for the Japanese mafia used to cut off a finger if you did something they didn't like or lied them. Usually pinkies were the first to go. You can see this in some Japanese cartoons too.
This was brought to US from immigrants and movies. I am not sure where/when it became a "cute" thing to do. Because two people who did not know each other well would not pinky swear. Definitely alteration after acceptance.