In South Brazil's Hunsrückischer dialect, there is a very known expression (even used by some portuguese speakers) which sounds approximately Das s mu schlimm (There is hardly any examples of written Hunsrückisch and I've never seen the sentence written, I am doing my best to write it as it sounds)
The expression is used when one is faced with a bad/difficult/sad situation.
Now, I can recognise that the Das s is probably a contraction from Das ist and schlimm is just like the Hochdeutsch word for bad. But I cannot find any reason for the mu to be there. A gut feeling tells me it is an intensity adverb, but I didn't find any word that would possibly be an explenation (maybe mehr but a shift from e to u seems odd) The documentation on this dialect is quite hard to find.
Where possibly could this sound come from?