verb (used with object), verb (used without object)
to make or become tough or tougher.

1575–85; tough + -en1

toughener, noun

harden, firm, strengthen, stiffen. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
Cite This Source Link To toughen
World English Dictionary
toughen (ˈtʌfən)
to make or become tough or tougher

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
Embedded in the amorphous portions of both proteins are two kinds of
  crystalline regions that toughen the silk.
If your housing policies are too modest, you toughen them up.
But the words on accompanying plaques toughen them up.
The measures would cap payments to large farms and toughen environmental
  standards while leaving the overall budget unchanged.
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