Originally a Sumerian adjective used to denote "evil" or "great," it has made its way into modern popular culture via books such as Simon's "Necronomicon" and developed into a term for "bad in a cool kind of way" or "cool in a baaaaad kind of way." It is synonymous with the street version of "bad" meaning favorable or exceptional. Also, "more cool than cool" and subsequently difficult for the unenlightened to pronounce.
The "X" is intoned as an exaggerated German or Arab CH, as in "Achmed."
What if we invite some Star Vampires to the Prom?" "Dude, that would be, like, totally XUL!!