1 : to become hard or firm or to make (something) hard or firm [no obj] ▪ It will take several hours for the concrete to harden (into a solid block). ▪ The gel hardens quickly. ▪ The presence of certain substances in the blood can cause the arteries to harden. [+ obj] ▪ These additives are designed to harden the steel. ▪ substances that can harden the arteries 2 : to become more definite and strongly felt or to make (something) more definite and strongly felt [no obj] ▪ Opposition to the goverment has hardened as news of further scandals has leaked out. [+ obj] ▪ The news has hardened opposition to the government. 3 [+ obj] : to make (someone) hard : to make (someone) less emotional and less likely to feel sorry for other people
▪ He had been hardened by his years of military service. 4 [no obj] : to begin to be or seem harsh, angry, serious, etc.
▪ Her voice/manner hardened as she spoke of her determination to win.