Originally an ad hominem attack against Jane Curtin by Dan Akroyd on Saturday NightLive during Weekend Update. At that time, it featured a "Point/Counterpoint" in which Jane made one point, and Dan would prefix his counterpoint with this phrase.
More recently, it has become sort of a ha ha only serious opener to such a counterpoint on Usenet, typically used amidst flamewars.
Jane, you ignorant slut! You know this needs no example!