The most sacred of vows. It is done when two people hook their right pinkys together and one says "I pinky promise...." followed by what they are promising.
This is not something to fuck around with. A pinky promise is serious. You don't break a pinky promise, that's just shitty...
...it's like kicking someone in the shin, which will happen to you if you break a pinky promise!
Jon pinky promised Bob that he would remember to give him a ride to the airport.
Jon forgot about taking Bob to the airport, and Bob missed his flight.
That afternoon Jon's wife, mother, father, and the secretary he was having an affair with all died in a fire during an open discussion about Jon's affair. Jon found all of this out, and then was kicked in the shin and mauled to death by a bear.
This all happened to Jon because he broke a pinky promise.