When you accidentally touch someone else's body in an inappropriate way, such as if your hand touches someone else's butt while walking. Often leads to embarrassment and awkwardness for both people.
*Naomi and Nicky walking along, Nicky's arm swings too far and hits Naomi's butt*
Nicky: Oh god, I'm sorry!
Naomi: Freudian Slap!
If it were to occur repeatedly:
"Stop Freudian Slapping me!"