(v. i. & t.) To grow or make tough, or tougher.

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