A granny slap happens when a grandmother approaches you and pinches your cheeks so hard that the skin literallyslaps 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.
The term Granny slapper is generally used to describe an old woman (Most commonly fat) Who is into making pornography with younger men. Granny slappers can be described as Skanky and/or slutty individuals for being notoriously dirty. This term can also be used to describe someone's Grandma as an insult.
"Bro, is that his your over there?"
"Yeah, why?"
"cause' she's a fucking grannyslapper"