This is an interjection taken from the Zora Neale Hurston novel, "Their Eyes Were Watching God" Many of the African American characters speak with very Southern Dialects. "'I God" is actually a shortened form of "By God" This term is often used in order to mock the dialogue of the novel. It can also be used to express one's god complex.
1.) We're gonna have a flood out here 'I God!
2.) I'd rather eat a bullet than ever read "Their Eyes Were Watching God" again, 'I God.