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1. To become more determined, uncompromising, stoic, resilient, strong-willed, etc. You've really got to toughen up if you want to make it in this industry. There's no room for cowards or pushovers! It was just a joke—toughen up, kid!
2. To cause someone to become more determined, uncompromising, stoic, resilient, strong-willed, etc. In this usage, a noun or pronoun can be used between "toughen" and "up." I know you think I'm hard on you, but I'm just trying to toughen you up before you go off into the real world on your own. I was really sensitive to criticism, but performing standup comedy toughened me up real quick. We're hoping this training course will toughen up the new interns.
toughen someone or something up
to cause someone or something to be stronger, more uncompromising, or more severe. A few days behind the service counter at the dis count store will toughen her up quickly. Having to deal with complaints toughened up the clerk quickly. She tried to toughen the skin on her palms up.
to become tougher, stronger, or more severe. She will toughen up after a while. You have to be tough around here to survive. You are going to have to toughen up if you want to play on the team.