To grin retartedly and/or drool while surrounded by extreme awesomeness because you can not express your joy/excitement/admiration for what ever that is around you with words.
Derived from the words "herp," meaning epic win, and "derp" meaning moment of retardation(according to some definitions).
Commonly occurs when listening to music or watching movies.
B:"It had me derping in herps!"
A: "Lucky bastard."