The story
Probably from dur, a British informal exclamation for “stupid” or from duh, an exclamation indicating faltering speech and assumed stupidity, glottalized for emphasis. Popularized by BASEketball (1998) and "Succubus", a 1999 episode of South Park. See also twerp possibly related to Welsh twp meaning fool. Derp was used in the movie College Rhythm (1934) to refer to a nerdy or awkward type of person.
Etymology from Wiktionary, CC BY-SA 4.0.