ero = Energy Regulatory Office.
http://dictionary.reference.com/search?q=ero
No results found for "eroi".

"ero"/"eroi" is not an English word, it's a bastardization of an English word. Japanese use messed English loanwords all the time. Also, it's not "ero(i)", it's エロ(い). Not the same alphabet.

As for "HENTAI" (all caps, written in Roman letters) it has gone back into Japanese with another meaning. It now means "This is what the Westerners call erotic, lol lol lol". Language is a living thing, it evolves.