This word is basically the word "what". Except for the fact that it has four "a"'s and no "t".
Whaaaa is a word to show confusion, shock, or just to show that you have no idea to what just happened. The word "Whaaaa" usually has a question mark behind it.
Paul: "I can't believe that just happened! Did you see that?!
Doug: "Whaaaa?"
Means white trash. It refers to someone who lives in a trailer park, drinks excessive amounts of beer, frequents Walmart often, and if all these things are true, even more "whashy".
Look arpt that mom getting wasted with her kids right there. She's so whashy.