It's just a more fun way to say oh my god! Or you can say ermahgersh if you don't feel like saying ermahgerd. You can talk with the 'E' and 'R' with almost any word. That's the only language me and my friend Joey speak to eachother. It's pretty fun!!
(Noun): Ermagerd is a term used when (on social media) you see a crazy post (or a spicy meme) and want to say "OH MY GOD" but don't say it right, or just can't spell.