diss on someone
dis(s) (on someone)(ˈdɪs...)
in. to belittle [someone]; to show disrespect [for someone]. (From disrespect.) Gary is such a complainer. All he does is diss.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.