Mos Def comes from two words, mos is short slang for "most" and def is short for "deft" which means skilled. Thus "mos def" is defined as "most skilled."
As coined by the legendary rapper himself, "Mos-Def" refers to a thing, person, experience etc which is "mos" (or most, hugely, a lot), "def" (excellent, awesome; NOT short for 'definitely').
Mos-Def's changing his rap name to "Yasiin"?? Don't sweat it, he's still the MOS DEF rapper out.... after Chuck D of course..