To diss is to disrespect; to treat, mention, or speak to rudely. It is a slang word, chiefly US, that came from, and therefore shares the same definition as "to disrepect"; to treat (someone) with contempt.
The City of Dis - sketch by Boticelli
Dis with one ‘s’ refers to the city of Dis, from Dante's levels of hell. The city of Dis is the level of hell to which all non-believers, who have not otherwise sinned greatly, will go.