1. just like saying "are you serious?" or "really?" or "for real?" when used by itself as a question
2. an alternative to "demonstrated existence", "proof", "evidence", etc.
3. (used to indicate strong agreement, assent, etc. or to simply state "yes, that is accurate information" aka "for real" or "affirmative"):
"William's mom has big ol' titties."
"Demo X she's got big ol' tittums!" (everyone save William laughs)
"Doesn't Nathan Vasher lay out Carlos Francis when they run that fake punt play in this game?"
see also: hells yeah, hell yes, damn straight, F*CK YES!
4. to demonstrate existence of
5. to prove
"Can you demo x the Heisman you won, Mr. Young?"
"Can you demo x you won the Heisman, Mr. Young?"
Mr. Young: "No. I don't have one. Y'all demo x'd the Heisman is a bunch of bull after you passed on me. That's like giving MJ's Finals MVPs to Tony Kukoc!"
6. demo x = "chill out"; demo = "chill", x = "out"
demo x = "simmer down"; demo = "simmer", x = "down"
"calm down", etc., etc.
***This definition is the only one that is not obviously intuitive. Demo x is a popular way of saying "chill out" to someone when that person is becoming too excited or nervous or agitated about something because since prior existence has been demonstrated regarding the subject at hand, the person doesn't need to get all worked up over it. It's been done. It's old news. Now move on.
Gregory (near whisper): "Elizabeth's cheating on me."
Robert (whisper): "What?"
Gregory (louder, more enunciated whisper): "Elizabeth's cheating on me."
Robert: "Demo X? Do you have any demo x?"
Gregory (markedly louder): "Demo X, Bobby! She demo x'd her infidelity when I walked in on her blowing Christopher at Christen's Christmas party for Chrissakes! DO I NEED ANYMORE DEMO X THAN THAT?!?! DO I??"
Robert (controlled, authoritative whisper): "Demo x, Greg! Demo the f*ck x, brah."