The American word meaning (1) to disrespect, abuse, or insult and (2) an act or instance of disrespect, abuse or insult was originally spelled dis when it emerged in the late 1980s. But diss has gained ground as an alternative spelling, and the two are now battling for ascendancy.
Dis makes sense if the word is indeed shortened from disrespect (the prevailing theory about the word’s origin), but anyone unfamiliar with dis may be tempted to pronounce it dizz. Diss‘s pronunciation is unambiguous.
In either case, the word is inflected dissed and dissing.
Both forms appear regularly—for example:
So perhaps the AP was just miffed that just a close partner organization would dis it in public. [Washington Post]
Romney’s camp did not acknowledge the diss in any of its public statements Sunday. [New York Daily News]
Jimmy Fallon Apologizes to Michele Bachman for Roots Dis [Us Weekly]
[A]fter the episode aired and Bachmann found out about the diss, she wanted an apology. [Los Angeles Times]
Delivering a dis, in other words, can be a nuanced art. [Wall Street Journal]
He’s quick to dismiss those who diss his audience. [Irish Times (dead link)]