Derp or Herp or herp derp is...
Hey, you just did something dumb. Well. That's derp (dumb).
A picture like this is Derp, because it looks really funny and embarrassing:
Its also just a funny/stupid sound, perfect for being silly and making fun:
I usually say derp like this:
I was walking down the street today and I tripped and fell. Derp.
Derp in this instance, Aka, I did something silly and mild, which is funny, haha.
So lets say, you, DQ, were doing a water change and forgot conditioner.
(Aka, oops, had a "brain fart" or simple mistake, how silly~!)
Derp is what you make it.