徒勞無功

- prove futile; beat the air; be a wasted effort; burn daylight; carry coals to New Castle; catch shadows; catch the wind in a net; come to naught; draw water in [with] a sieve; end in smoke; fish in the air; flog a dead horse; hold a candle to the sun; labor to no purpose; make a futile effort; make useless trouble; milk a hegoat; much ado about nothing; not to succeed after trying hard; paint the lily; plough [sow] the sands; plow the wave; use [exert] vain efforts; wasted effort; work in vain; work to no avail