noun
the process of becoming or making something hard or harder
verb
present participle of the verb 'harden', which means to make or become hard or harder
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The process of hardening steel involves heating and then cooling it to increase its strength.
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I heard that the new regulations are really hardening up on security around here.
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Dude, you need to stop being so soft and start hardening up if you want to succeed in this industry.
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His heart was soft at first, but after going through so much pain, it started hardening against the world.