What Is This X You Speak Of?

Also seen as "What Is This X Of Which You Speak?" and "X? What means this X?",

A Stock Phrase that today is used in conversation to express a sarcastic dismissal of a concept that another person takes for granted, or to imply that the topic of the sentence -- X -- is unknown in context of the discussion. For example:

Which implies that NSA is ignoring or ignorant of the Constitution and its limits on government power.

The origins of this phrase are obscure; it appears to be rooted in a generalized memory of cliched lines from bad SF movies from the middle of the 20th Century. The prototypical scene involves aliens who speak English perfectly, yet still need a handy Earthman to explain ordinary concepts to them -- even concepts one would expect would have obvious counterparts in their own culture or biology. It also seems to be related to the archetypal Jungle Princess's inquiry, "Kiss? What is 'kiss'?" However, examples of the usage can be found at least as far back as the 18th Century.

It is really in this context that All The Tropes is interested in the phrase. Since its explosion into the memesphere as a sarcastic Snowclone the modern use of the phrase can be found just about anywhere -- we're far more interested in the original use of the phrase and its variants. (If you've got a particularly humorous example of the modern use that you still want to share, though, put it on the Quotes subpage.)

Not to be confused with "What Is This, X?"

Literature
""Look, sorry, are we talking about the little white furry things with the cheese fixation and women standing on tables screaming in early sixties sitcoms?"
 * The Hitchhiker's Guide to the Galaxy has a variant instance, which at least one linguistics blogger suspects may be the trigger for modern (over-)usage of the phrase:

Slartibartfast coughed politely.

"Earthman," he said, "it is sometimes hard to follow your mode of speech. Remember I have been asleep inside this planet of Magrathea for five million years and know little of these early sixties sitcoms of which you speak.""

""'Father, you amaze me! What is this love of which you speak? I neither know its nature, nor if I felt it, why I should conceal the sentiment.'""
 * The Monk, by Matthew Lewis, a Gothic Horror novel written in 1794:

""My little man, where do you come from? What is this 'where I live,' of which you speak?""
 * The 1943 English translation of The Little Prince by Antione de Saint-Exupery tweaks the original French into this form:

Live Action Television
"Kara: "Brain" and "brain"! What is "brain"?"
 * Star Trek the Original Series, the infamous episode "Spock's Brain":

New Media
"There has been a lot of net discussion about "toilet paper" recently. Just what is this "toilet paper" of which you speak? Where can I find it?"
 * The earliest known Internet example of the Snowclone usage can be found in a post to net.misc on August 24, 1983: