The act of being a complete and total fool, jackass, or moron. Doing acts that are not considered proper in public, or even in private. Only the dumbest of people can achieve the level of hardcore derping. A prime example would be when Simon Cowell completely degrades a great singer, that is hardcore derping.
Bob walks in. "Man you were Hardcore Derping it at the party tonight". John throws beer can at Bob. "Derp."