My friend sack tapped me

I have a friend of mine who is about 19 years old. We were just having a walk in the garden, she was wearing sports shorts and a jacket.

We were just having a talk about school days and she suddenly said "SACK TAP!" and hit me in the balls with her backhand so hard I literally went on my knees. I had the hardest erection ever with those white legs in my view.