The Collaborative International Dictionary of English v.0.48:

Toughen \Tough"en\, v. i. & t. [imp. & p. p. Toughened; p. pr.
   & vb. n. Toughening.]
   To grow or make tough, or tougher.
   [1913 Webster]

WordNet (r) 3.0 (2006):

toughen
    v 1: make tough or tougher; "This experience will toughen her"

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "toughen":
   acclimate, acclimatize, anneal, be tough, beef up, brace, brace up,
   buttress, calcify, callous, case harden, confirm, cornify, develop,
   endure, firm, fortify, fossilize, gird, hang tough, harden,
   indurate, invigorate, lapidify, lithify, nerve, ossify, petrify,
   prop, refresh, reinforce, reinvigorate, restrengthen, season,
   shore up, steel, stiffen, strengthen, support, sustain, temper,
   undergird, vitrify