[gbh444g]

I'm not a linguist and I don't even know any greek (other than the alphabet I've memorized once), but if "epi" means "above" and "ousia" means "essense", then the meaning of "epiousios" trivially follows.

https://en.wikipedia.org/wiki/Ousia

[cratermoon]

The article addresses that, and suggests the problem is that in most contexts 'epi' loses the i in compound form, e.g. eponym.