The act of grabbing the excess skin of your ball sack, lifting it up like elephant ears and slapping a girl in the face with your dick!
Branham - Thompson, what happen with that chick.
Thompson - She sucked at giving head so I gave her a trunk slap.
When your girlfriend and/or wife is post sex cuddling and you roll over and whack her in the eye with your penis.
"She looked so peaceful, so I trunk slapped her!"