David Weinberger expressed great puzzlement over this ubiquitous Texas term, when in Austin for SXSW.
Fortunately, Prentiss Riddle comes to the rescue with a pronounciation guide, a definition from a Spanish-language guide to Aztecisms, and links to the wikipedia entry on Nahuatl (the language used by the Aztecs, from which we also get the words "chocolate" and "tomato.")
Thanks, Prentiss.