1
general::
Hostile or negative toward, ill-disposed to. For example, All the reviewers were down on this play, or Ever since he was injured, he's been down on skiing. [Mid-1800s] Also see DOWN ON ONE'S LUCK.
American Heritage Idioms
2
general::
Phrase(s): *down on someone or something
against someone or something. (*Typically: be ~; get ~.) • You’ve been down on us all lately. • I’m down on computers lately. • Everyone sure got down on fast food.
McGrawhill's American Idioms And Phrasal Verbs