pinky swear

n.

An eternally binding act of hooking pinky fingers together in an attempt to seal the deal of a promise that has been made.

The pinky swear originally indicated that the person who breaks the promise must cut off their pinky finger.

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.

Also known as pinky promise.