1 [with object]British jeer loudly at (someone performing or speaking in public) in order to express disapproval or to distract them:opponents barracked him when he addressed the opening parliamentary session
2 [no object] (barrack for) Australian/NZ give support and encouragement to:I take it you’ll be barracking for Labour tonight?
late 19th century: probably from Northern Irish dialect
barrack in other Oxford dictionaries
Definition of barrack in the US English dictionary