1dead·ly 
/ˈdɛdli/ adjective
dead·li·er; dead·li·est /ˈdɛdli/ adjective [or more deadly; most deadly] 1 : causing or able to cause death
▪ deadly weapons ▪ the world's most deadly snake ▪ a more deadly form of the disease ▪ They launched a deadly attack. ▪ Officers are allowed to use deadly force if necessary. — compare deathly 2 : extremely accurate and effective
▪ She shoots with deadly accuracy. ▪ a basketball player with deadly aim 3 always used before a noun : extreme or complete
▪ A deadly silence followed his question. ▪ The two gang leaders are deadly enemies. ▪ He spoke with deadly seriousness. 4 informal : very boring
▪ The lecture was pretty deadly.
▪ deadly weapons ▪ the world's most deadly snake ▪ a more deadly form of the disease ▪ They launched a deadly attack. ▪ Officers are allowed to use deadly force if necessary. — compare deathly
▪ She shoots with deadly accuracy. ▪ a basketball player with deadly aim
▪ A deadly silence followed his question. ▪ The two gang leaders are deadly enemies. ▪ He spoke with deadly seriousness.
▪ The lecture was pretty deadly.
— dead·li·ness noun [noncount]
▪ the deadliness of the disease
▪ the deadliness of the disease
synonyms deadly, mortal, fatal, and lethal mean causing or able to cause death. deadly describes something that is very dangerous and likely or able to cause death.
▪ deadly diseases ▪ deadly weapons mortal describes a wound or injury that has already caused death or will soon cause death. ▪ She received a mortal wound and died the next day. fatal describes something that has actually caused a person's death. ▪ The disease is sometimes fatal. ▪ The wounds later proved to be fatal. lethal usually describes something like a poison that is used for the purpose of destroying life. ▪ a lethal gas
▪ deadly diseases ▪ deadly weapons mortal describes a wound or injury that has already caused death or will soon cause death. ▪ She received a mortal wound and died the next day. fatal describes something that has actually caused a person's death. ▪ The disease is sometimes fatal. ▪ The wounds later proved to be fatal. lethal usually describes something like a poison that is used for the purpose of destroying life. ▪ a lethal gas
