1.: A slang term for "most definitely."
2.: A rapper.
3.: Deaf - this useage is the least common, mostly by people who didn't understand it in the first place or mispronounced.
1.: "Am I going to the party? Mos def!"
2.: "Mos Def is one of the best rappers of our time."
3.: "After that concert, they're mos def."