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!"