A term used when a person wishes for a surprising or astonishing statement to be repeated, or simply to show their surprise at said statement. To accurately model the timbre of the phrase, omit a high pitch during the last portion of the phrase in addition to stretching the "what" as long as deemed necessary by the user: Say (In high pitch) Whaaaat?!!
Modern version of the grandparents' phrase "I beg your pardon." Words better when talking to friends, but not so much when talking to teachers or bosses.
1: You wanna gohang later?
2: Say what???
1: I said you wanna go hang later?
2: You know it!