A fan of the New Orleans Saints. Originated from the fan generated chant in the late 80's, somewhat decent run of success, for the New Orleans Saints. See example saying.
Who Dat? Who Dat? Who Dat say dey gonna beat dem Saints?!
Ghetto semantics, used throughout but most common in the ghettos of the Dirty South, meaning "Who is that?". Properly expressed as "Who dat is", however "Who dat" is more popular as it requires considerably less mental effort.
Person 1: "Who dat is? That nigga on noe hea'r block again."
Person 2: "He hustlin' foe shaw, look at that fool."