Dis-
Lighter ("Historical Dictionary of American Slang") states its origin is Black, earliest citation found is 1982, an interview with "Crash Crew" in Rap, defined as "to disparage, to belittle." It appeared in television in "Miami Vice" in 1984, where it (dissed) clearly was used as 'disrespect'. By 1989, it appeared in "Village Voice."
By 1990, it appears in print as a noun- "It's a "dis."
(Many examples quoted, vol. 1 p. 605)

Just speculating, no evidence- It could come from the old use of 'dis' (OED, 1925) as a word for disconnected - not with it, feeble-minded- when applied to someone, it certainly is a disparaging way of speaking of him; most disrespectful.
A few students of language would not regard 'dis' as slang but as an abbreviated form.

Roget's Thesaurus, under synonyms for disrespect, lists disparage. Under synonyms for contempt appears disparage. Etc., etc.

To an old time printer, 'dis' meant unsorted (with regard to type).