Dissing is a slang term derived from the word disrespecting. The verb "to dis" originated in African American Vernacular English, though its use has become widespread among people of many racial and social backgrounds. A diss can occur by either spoken word or song lyric
(Slang, also dis) show disrespect toward another person; insult someone, belittle another person, disrepect; treat without respect: "I went out of my way to help her but she dissed me anyway."
to put (someone) down, or show disrespect by the use of insulting language or dismissive behaviour, perhaps originally short for disrespect or disparage
If someone disses you, they criticize you unfairly or speak to you in a way that does not show respect. He believes that his records speak for themselves and ignores those who diss him. dissertation. to say unkind things about someone you know