tough ‧ en /ˈtʌf ə n/ BrE AmE ( also toughen up ) verb [intransitive and transitive]
[ Word Family: adverb : ↑ toughly , ↑ tough , ↑ toughly ; verb : ↑ toughen ; noun : ↑ toughness ; adjective : ↑ tough ]
to become tougher, or to make someone or something tougher:
toughened glass
Three years in the army toughened him up.