Regardless of it's origin and how long it's been around, the term "Who Dat" along with the rest of their slogan is a mockery of the English language and is about as unprofessional as it gets. The fact that the league has embraced this nonsense is a joke. You can't celebrate too much after a big play because it's not professional, but you can butcher the English language and everything is fine.
Try going to an interview around here for any respectable job and speaking like those buffoons. You'll be laughed out of the building.
"I be lookin fo dis job cuz I wan' make dat cash!"