- 1More deadly and deadliest are the usual forms. You can also use most deadly. causing or likely to cause death synonym lethal a deadly weapon/disease deadly poison The cobra is one of the world's deadliest snakes. The terrorists have chosen to play a deadly game with the civilian population.
- 2 [only before noun] extreme; complete We sat in deadly silence. They are deadly enemies (= are full of hatred for each other).
- 3extremely effective, so that no defense is possible His aim is deadly (= so accurate that he can kill easily). Snipers can shoot with deadly accuracy.
- 4(informal) very boring The lecture was absolutely deadly.