A promise made between two people, more common among young people or children, sealed by shaking hands -- except only with pinkie fingers. The idea is that if the deal is broken, the culprit will lose his or her pinkie finger.
The pinkie or pinky finger is the smallest, or farthest from the thumb. Also spelled pinky swear.
"I swear I won't tell anyone!"
"Do you pinkie swear?" (holds out pinkie finger)
"Pinkie swear." (hooks pinkies together and shakes)