Waterslide surprise 

The unexpected feeling of a person's genitals on another person's back, sometimes when said persons are on a waterslide together.

I gave him the waterslide surprise

Waterslide surprise by jacinto obnagon March 24, 2003

Waterslide surprise 

When a male is travelling down a water slide and somehow the genitalia suddenly reverse direction and become crushed under the body. The genitalia then proceed to experience extreme friction for the rest of the ride. This is somewhat surprising.

Ricardo was travelling down the slide and suddenly he had an expression of anguish because of waterslide surprise

Waterslide surprise by Steven Brinks December 6, 2006

Waterslide surprise 

When like ur jus' chillin' on da max and some random guy is jus' like starin a hole through ur head and has a dazed look on his face and the jus' let's it shoot if u get my drift.

Omg, he jus' had a waterslide surprise

Waterslide surprise by Mandi rikitu May 7, 2003