bag on (someone)

Definition

Slang

  • verb to insult or deride. A term from black street slang from the 1990s, it is probably adapted from earlier slang senses of bag as in to dismiss, abandon, etc. but is unlikely to be related to the similar Australian use of ‘bagging’ to mean criticism.
http://www.dictionarycentral.com/definition/bag-on-someone.html