to waste one's breath; to talk in vain

to waste one's energy or thoughts on a futile effort; to scheme or worry for nothing

To exert effort without getting any results; specifically a colloquial abbreviation of the folk riddle [[瞎子点灯|瞎子點燈|xia1 zi5 dian3 deng1]]—bái fèi là, meaning like a blind person lighting a candle, the effort is wasted.