plough the sand
plow the sands
To do something that seems futile. I know that I'm just plowing the sands by telling you to stay away from that boy, but I'm your father, and I don't want to see you get hurt. That couch will never fit up the steps—tell them to stop plowing the sands!
plough the sandlabour uselessly.
Ploughing the sand has been a proverbial image of fruitless activity since the late 16th century.