To be absolutely opposed to something

I really hoped my son would become a football player, but he was dead set against it. As it turned out, he became an internationally known concert pianist

Speravo proprio che mio figlio diventasse un giocatore di football, ma era totalmente contrario a questo sport. Alla fine è diventato un pianista di fama internazionale

My boss was dead set against giving anyone a raise, so I resigned and changed company

Il mio capo era totalmente contrario a qualsiasi tipo di aumento di stipendio, perciò mi sono licenziato e ho cambiato lavoro

According to one theory, this phrase may come from the industry field, where a machine is said to be "dead set" when it is fastened so that it cannot be moved. Then its meaning was transferred to hostility against some person or course of action. According to another theory, set against means opposed to since the 15th century