It's a multi-usage-word (similar to meep).
It can be pronounced however you like, and replaces any emotion, salutation, verb, noun or adjective you want it to do.
To Moep also means, to shout out "Moep".
"Moep! ^_^ How are you?"
"This is really moep."
"Beat the f*cking moep outta him!"
"I just wanna moep now."
"I feel sick. Moep."