toughen (v.)

• acclimate
• acclimatize
• anneal
• brace
• buttress
• calcify
• callous
• confirm
• develop
• endure
• firm
• fortify
• fossilize
• gird
• harden
• indurate
• invigorate
• lapidify
• nerve
• ossify
• petrify
• prop
• refresh
• reinforce
• reinvigorate
• restrengthen
• season
• steel
• stiffen
• strengthen
• support
• sustain
• temper
• undergird
• vitrify