DESCRIPTION:
I STRONGLY recommend you read this first if you don't know the origins of the pinky swear. The song doesn't make sense otherwise.

Yubikiri, which means pinky swear or pinky promise, literally means to cut off one's finger. This is because in ancient Japan (I don't know the era), a pinky promise was when a prostitute fell in love with one of her clients and cut off her pinky finger to give to him as a symbol of her love. That is why Miku is dressed as a prostitute in this video - it is to connect the song with the original meaning of a pinky swear.