2. n: One who is very stupid, or who fails a lot at life.
From the words "hella" meaning: "a lot of, very, really," and "derp" which is often said after one has failed, pretty much meaning: "Something that was stupid/dumb,"
2. Person A: "Dude, I walked into a synagogue the other day wearing a swastika t-shirt and shouting "Hail Hitler!" lol Everyone looked at me like they were going to kill me!"
Person B: "...You're such a helladerp."