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
by jacinto obnagon March 24, 2003
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
by Steven Brinks December 02, 2006