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To make hard or harder.
(verb-transitive)
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To enable to withstand physical or mental hardship.
(verb-transitive)
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To make unfeeling, unsympathetic, or callous: "To love love and not its meaning hardens the heart in monstrous ways” ( Archibald MacLeish).
(verb-transitive)
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To make sharp, as in outline.
(verb-transitive)
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To protect (nuclear weapons) by surrounding with earth or concrete.
(verb-transitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.