Originated in the 90's. Made by a poet/rapper named Mos Def. Def means Definate which was first used by Run-DMC. Around the 90's there was Def Jam, a company that influenced Rap, Comedy and many other things. Also a video game featuring people signed on the Def Jam label.
come on pal! i'm a white british kid and i know def jam didn't originate in the mid 90's and has fuck all to do with mos def. it (was) a legendary record label, see public enemy, beastie boys, ll cool j and bunch of other old skool legends
...err, i think slick rick used to be there too. btw, it's bit wank now, like most hip hop