A granny slap happens when a grandmother approaches you and pinches your cheeks so hard that the skin literally slaps against your face when released. It is usually accompanied by much cooing and statements that include but are not limited to: "Oh, you've grown so tall," or, "You're my little sugar bunny!"
John: Did you see that mark on Jack's cheek?
Jill: Yeah, he totally got granny slapped yesterday. His sister told me all about how his skin snapped into place.