Another, lesser known name for a special person. Meant to be as offensive as the word retard. Though it can be used as universally as the word "fuck" such as "Quit being a such herp-a-derp" or "go be a herp-a-derp somewhere else". however the word can be butchered to be simply "herp" or "derp". Although some "herp-a-derps" will reverse the word and say "Derp-a-herp". Saying "Herp-a-derp" as a question changes the meaning to "A retard?" or "That Retard?".
A variant of Herp Derp, Herp da Derp is:
1.) An expression when someone says something obvious.
2.) An expression for when someone says/does something stupid.
3.) A random expression usually used during an awkward silence, but can be used out of complete randomness; can also be used as a greeting.