Who Dat Huh?

Is it an acronym? Devil as Texan?
Is it an acronym? Devil as Texan?

Commoners of America: What does “Who Dat?” mean? Who asks it? Is it a genuine question being asked of me, or is its rhetorical purpose to underscore and emphasize something? Why is this emblazoned everywhere, such as on the shirt of the Best Buy employee who assisted me in last night’s purchase of a washer and dryer? I just don’t have an answer prepared if someone asks me, and I hate to be caught unawares.