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1.
(interjection) An interjection used among Pokémon fanatics to replace "Oh my God".

The reason why Arceus replaces God is simple: Arceus is the Pokémon responsible for the creation of the Sinnoh region, and possibly the whole universe (Pokéverse, if you may) according to myths around the world of Pokémon. His legend dates back to Generation 4 (Diamond, Pearl, and Platinum). Arceus is also called "The Original One".

Arceusism is a polytheistic belief system. According to said myth/religion, Arceus is said to be born from an egg, emerging from a vortex, and then created the Legendaries of time, space, and antimatter: Dialga, Palkia, and Giratina, respectively. When mankind started inhabiting Sinnoh, he created three Pokémon to give humans knowledge, emotions, and willpower.

The term, in real life, is used as jargon among fans, as reversed verisimilitude to the Pokémon universe. It is also used online where "Oh my God" might provoke a number of religious and/or sensitive people.
"I just got the new Pokémon game!"
"Oh my Arceus! Can I see?"
by otaku sempai kun November 04, 2013
 
2.
1. A phrase created by Pokéfans to replace Oh My God because they have sold their souls to the Pokés and must worship their lord and master Arceus, who has yet to grace the American nor the Waponese audience as of yet. Arceus is only know of through leet skillz. When the day finally comes that their satanic God comes forth from the Spear Pillar to the hands of fat boys playing their Nintendo DSs, the world will finally burn in the fires of hell.

2. An exclamation said when you fall flat on your fat white ass and have played too much Pokemans. Also cried out when you get asswhipped with a birch rod] for "trading" *cough cough little theives cough cough* Pokémon Cards when you should be learning how to count to thirteen and spell your name. Related to Arse.
1. Oh My Arceus! What in Girantina's Realm is he thinking?

2. "Oh My Arseus! Thwat hwurt! Gwiv mee back my Pokeman cards!"
by Brân Moroedd Lledyeith February 11, 2008