A fork of Rural Dictionary
Herpa derpa is nonsense babble. When someone says something that makes no sense and sounds absolutely stupid you can respond with "Herpa derpa" as a quick painless way to avoid further conversation with whoever makes an idiotic statement. People use it to point out a stupid or ignorant remark or behavior.
"I'm going to jump off this cliff and fly to the moon with my arms."
"Herpa derpa?"