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®

toughened
     adj 1: physically toughened; "the tough bottoms of his feet" [syn:
            {tough}] [ant: {tender}]
     2: made hard or flexible or resilient especially by heat
        treatment; "a sword of tempered steel"; "tempered glass"
        [syn: {tempered}, {treated}, {hardened}] [ant: {untempered}]