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?”