To make or become tough. See Synonyms at harden.(verb-transitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "toughen" in a sentence
"You gotta fuckin toughen up with the cold and fuckin dealin with rugged not you average fuck around kinda guy."
"Eventually, the skin will "toughen," and your child won't feel as much discomfort."
"It's really cute--but you could "toughen" it's image up to make it more "Terp" appropriate."