This is a sexual move used mostly by the Vulpsie. Made popular by her in the late 90's in the Bay Area gay sex scene. It's really rowdy stuff not for the faint of heart.
It involved the use of a butt plug that has been inside several butts.
Amy pulled out her classic elephantslide last night, and today I'm walking like Yosemite Sam
The Rusty Slide whistle begins with a woman furiously blowing into their male counterparts asshole, while simultaneously tugging his johnson. This act replicates the steps required to play the slide whistle, thus warranting the name.
"Yo, last night Justine busted out the rusty slide whistle, we made some sweet music"