Street voices
"This is actually said with a dramatic and emphasized 'a', so it'd be 'say whaaaat?!' And it's typically used to express disbelief or shock. For example: 'You won a thousand dollars on that casino app?' 'Say whaaaat?!'"
What it means
Expression of disbelief or shock, often drawn out with emphasis on the 'a' for dramatic effect.
Usage examples
"You won a thousand dollars on that casino app? Say whaaaat?!"
"They're getting back together after all that drama? Say whaaaat?!"
"He ate the whole pizza by himself in ten minutes. Say whaaaat?!"