Derp is a stupid word, and I don't like it. It's used a lot in pop culture, and proliferates online, so I feel obligated to define and explain it.
"Derp" is a word used to explain disgust with someone's alleged stupidity or ignorance or to signify one's own behavior in the same vein.
Know Your Meme explains, "Derp is an expression sometimes used online to signify stupidity, much like the earlier forms of 'duh' and 'dur.'"
My generation is fond of it. We used to say, "Uh, doy!" when we were kids and then thump our chest, to mime the mentally handicapped. Derp has the same connotation. Sometimes the phrase is elongated to "herp a derp."
The word has made its way into Saturday morning cartoons. When an animation error made a pony on My Little Pony: Friendship is Magic "wall-eyed" the show's (notorious) adult, Internet-savvy, fans named the pony "Derpy." The pony is now a main character... and named Derpy Hooves.
Man, people are going to be mad about this one.
Derp, Urban Dictionary
Derp, Know Your Meme
Derpy, My Little Pony wikia