A cardy is a piece thin piece of cardboard rolled onto a cylinder, and used a filter in marijuana cigarettes. Its purpose is to prevent on inhaling ash and plant matter while toking particularly in the case of blunts and fat marijuana cigarettes. The use of a cardy also prevents the formation of a roach (see roach definition).
"roll a cardy in that joint man"
Essential northern clobber
"It`s a bit nippy outside, our Terry. You`d better pop yer cardy on if yer goin` out"