When a girl is performing felatio, the guy pushes too hard on the back of the girl's head and she slides up the shaft gagging and blowing out like a slide whistle. This is usually combined with a bit of vomit and/or saliva, making the shaft slippery.
I got pissed that this chick wouldnt deep throat, so I slippery whistled her, and now I got puke all over my boxers.
When a male and female are getting intimate and the girl is giving the guy a hippity handjob. Then the female, without the male's knowlege, hops on top for a slippery suprise, often making a slide whistle noise.
Hey guys, that bitch just gave me a Slippery Whistle!
I was shocked when that girl gave me a Slippery Whistle in bed!