Finally! I can put useless trivia to use!

It was first used in a Doritos / Frito Lay commercial series by the master of idiocy Chris Elliott!

It was a series of uncomfortable commercials from the nineties. Chris would come into a room and offer his Tostitos as a bribe for something. The guy eating the bribe nachos would then reply sarcastically "No, not really" - as if he was saying "yes, of course, really, duh!". And you are left blinded by the DOUBLE Sarcasm. Brilliant.

Added: Evidence! Link