When a methhead does something f'd up. (or messed up)
Meth-heads by nature have nil, or at best, limited teeth left.
Hence the pronunciation of "Meffed Up".
Can be used to describe a person's appearance, or an egregious situation, the term can be used quite broadly and still remain effective in meaning.
Did you see that lady back there... (grimaces/smirks) "Dude, she was all meffed up!!"