meaning something which is majorly bad. It can be used in a similar connotation to boring or shit to describe an activity, its just a much better word to use :)
Also is something is not totally bad it can just be gay bar, ie not good
gay bar lame shit boring gay uncool
i.e. gosh our party has been cancled 2nite cos timmos mum sed he cant have it at him house, this is BULK gay bar shes ruined all our plans.