Source : Webster's Revised Unabridged Dictionary (1913)

Toughen \Tough"en\, v. i. & t. [imp. & p. p. {Toughened}; p. pr.
   & vb. n. {Toughening}.]
   To grow or make tough, or tougher.

Source : WordNet®

toughen
     v : make tough or tougher; "This experience will toughen her"