To labour in vain/ to plough the sands/ "to sow among thorns"
In a question of advice: He has taken trouble for nothing. His labor will be reduced to impurity.
("For thus says the Lord to Judah and Jerusalem, Break up your fallow ground, and sow not among thorns" - Jeremiah 4:3)
Translation
Expression (Hebrew)
זָרַע אֶל קוֹצִים
Definition (Hebrew)
בְּהַשְׁאָלָה בִּמְלִיצָה: טָרַח לְחִנָּם. עֲמָלוֹ יָרַד לְטִמְיוֹן.
("כִּי כֹה אָמַר ה' לְאִישׁ יְהוּדָה וְלִירוּשָׁלִַם ,נִירוּ לָכֶם נִיר וְאַל תִּזְרְעוּ אֶל קוֹצִים"- יִרְמְיָה ד ג)