What Is 'Derp'? Is It an Online Insult?

Yes, 'derp' is a modern jargon expression for saying 'stupid!'.  Use of the word derp is very common in online gaming, Reddit commentaries, and Facebook posts.  Whether you see it spelled as 'derp' or 'durp', the meaning is the same: someone is getting insulted.

About.com explains more about derp here...