A word invented and developed in England by two sixteen year olds in 2011, on 9th June, at 21:13. It was inspired by Peter Griffin, and may have several different ways of spelling, but the correct way to write it is mnwehh, with two h's.
Mnwehh is used to express irritation, annoyance, frustration, exasperation, refusal to do something you were told to do, hopelessness, and other alike emotions. It's one of those words that doesn't really have a meaning but is used to show how you feel. Much like meh, which was also an inspiration for mnwehh.