it means.... nothing. :L like 'mrr' etc.
'myumyumyu' is actually three 'myu' put together, myu would be pronounced as mew, like a cat. And i said that kaws i was using pointless non-word words to 'show' that i was thinking :L
i would say that i use that non-word word combo often, but i dont even have 20 posts here yet so :L