Working very hard is spoken of as if they were to use all their strength until they had no more strength. The phrase "for nothing" means that they would get nothing from working so hard. Alternate translation: "You will work very hard in vain" or "You will work very hard, but you will not receive anything good from working so hard" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-idiom]])
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