Wiktionary
vb. (cx informal English) To swear with the pinky fingers entwined.
Wikipedia
To pinky swear, or to make a pinky promise, is the entwining of the little fingers ("pinkies") of two people to signify that a promise has been made.
In the United States, the pinky swear has existed since at least 1860, when Bartlett's Dictionary of Americanisms listed the following accompanying promise:
Pinky, pinky bow-bell, Whoever tells a lie Will sink down to the bad place And never rise up again.Pinky swearing presumably started in Japan, where it is called and often additionally confirmed with the vow . The gesture may be connected to the Japanese belief that soulmates are connected by a red string of fate attached to each of their pinkies.
In Japan, the pinky swear originally indicated that the person who breaks the promise must cut off their pinky finger.Sundem, Garth “10 Mundane Traditions with Strange Origins” 25 July 2011
HowStuffWorks.com
<http://people.howstuffworks.com/culture-traditions/cultural-traditions/10-mundane-traditions-with-strange-origins3.htm> February 2016 In modern times, pinky swearing is a more informal way of sealing a promise. It is most common among school-age children and close friends. The pinky swear signifies a promise that cannot be broken or counteracted by the crossing of fingers or other such trickery.