Superlative form of hard: most hard. Most rigid or most difficult.(adjective)
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Use "hardest" in a sentence
"Why on earth the Research Centers with the skills and academic abilities are being hit the hardest is a sign of politics in NASA more then anything else."
"Mr. Bush writes about his response to the 2001 terrorist attacks against the United States and what he calls the hardest decision any president can make - deploying troops in combat."
"Sean White, Brandon League and David Aardsma each pitched an inning in relief after Brown pulled Fister, which he called his hardest decision of the night."