Popularized by the movie, "The Heathers", this phrase is said in response to a request one deems impossible or absurd (much like being gently fucked with a chainsaw). It's basically a way of showing someone how ridiculous what they've just asked sounds.
Ex.
Veronica: “Why can’t we talk to other kinds of people?”
Heather Chandler: “Fuck me gently with a chainsaw. Do I look like Mother Theresa?”
Dylan: "Hey Nick can I borrow your car once its out of the shop?"
"Fuck me gently with a chainsaw. You crashed it just last week!"
The most powerful weapon in all existence. It has been known to cause multiple emotions in humans, the two most common being fear and sexual desire. It is thought that chuck norris himself crafted this destroyer of worlds. Photographs and drawings of the divine weapon must have an explosion photo shopped in the background so that the paper doesn't burn up from the awesomeness.