Possible Clues:

• Become hardened
• Overturned university in this matter to accept toughening up
• Become hard in river
• Hurt, Jack lost temper
• Urn thrown inside that is to harden
• Jack released from harm displays temper
• Acclimatize
• Become less sensitive (to)
• Accustom (someone to)
• Get used to being in the river