From the word "epileptic" wherein people mostly suffer from convulsions and/or lack of consciousness when exposed to bright rays of light for long periods of time.
"Epicleptic" is used to describe presentations such as artworks, games or animated feature films that are too high in brightness and/or high in color saturation therefore having the appearance of something looking "too friendly" or "too happy". Instead of using epic fail, we use "epicleptic" to describe these.
Dude, have you seen that new cel-shaded "Happy Tree Friends" game? It was really epicleptic, it almost made me puke.