1) Usually spoken with a drawn out emphasis on the last syllable; mofoshee is used to replace a combination of "mother f***er sh!t" when one needs to express themselves quickly.
2) A response to something that is though as being unbelievable, or a mistake
1) A dogg, let me holla at cha' mayne', we gon' go do dat mofoshee right now yo'
2) Person A: Dogg, I just bumped dirtys with your girl.
Person B: Mofosheeeeeeeeee, you ain't do nothin'