"derpy" is a third-removed phrase that is short for herp-derp, which itself is short for hurf-durf, which came from a Metafilter comment. The original usage is

"derpy" is a third-removed phrase that is short for herp-derp, which itself is short for hurf-durf, which came from a Metafilter comment. The original usage is something like "HURF DURF ${oblivious_statement}" and it meant to denote simplistic cluelessness. Apparently the connection to My Little Pony is that there was a dumb character named "Derpy Hooves". Some people took this character to be mentally retarded, and so "derpy" is seen by some as pejorative term for the mentally retarded. On Reddit, some people use "derp!" to indicate they had a temporary lapse in ability to understand something.