Quick answer
Injustice is seed, and the harvest is sorrow, the footnote reads "trouble," for the sower himself. And the rod he swings has a shelf life: the power behind oppression fails before oppression finishes its plans.
Read both lines
The first line plants the Bible's most durable law, sowing and reaping, in the field of iniquity: the man scattering injustice is farming, and the crop comes up in his own life. The second line watches his instrument wear out: the strength behind the rod is mortal, the fear that backs it thins, and the LORD's government is patient. Together the lines dismantle oppression's two illusions, that consequences can be outrun and that force can be permanent. Paul carried the law forward without adjustment: whatever a man sows, that he will also reap (Galatians 6:7). The oppressed are quietly handed the long view: the rod you are under is already failing.
Related: Proverbs 11:18 · Galatians 6:7–8 · Psalm 125:3
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.