If you are racked with pain or you feel nerve-racked, you are feeling as if you were being stretched on that Medieval instrument of torture, the rack. You rack your brains when you stretch them vigorously to search out the truth like a torturer. “Wrack” has to do with ruinous accidents, so if the stock market is wracked by rumors of imminent recession, it’s wrecked. If things are wrecked, they go to “wrack and ruin.”
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