deadly

deadly

I

dead‧ly¹ /ˈdedli/ adjective1.  VERY DANGEROUS likely to cause death
   SYN  lethal:
    a deadly poison
    a deadly weapon
    deadly disease/virus
2. deadly enemy someone who will always be your enemy and will try to harm you as much as possible:
    The two rapidly became deadly enemies.
3.  COMPLETE complete or total:
    We sat in deadly silence.
    He was in deadly earnest (=completely serious).
4.  VERY EFFECTIVE causing harm in a very effective way:
    She hit the target with deadly accuracy.
5.  LIKE DEATH  [only before noun] like death in appearance:
    His face had a deadly paleness.
⇨ seven deadly sins at sin¹(1)
     

THESAURUS

   ▪ poisonous containing or producing a substance that is likely to kill you, or make you very ill: poisonous gas | poisonous snakes | The plant is poisonous to humans and animals.
   ▪ toxic toxic chemicals and waste are poisonous to people and the environment: Crops were sprayed with highly toxic chemicals. | Lead is toxic to humans. | The site was used for the disposal of toxic waste.
   ▪ noxious [usually before noun] formal noxious substances and gases are poisonous: Vehicles pollute the air with noxious fumes | The soil may be contaminated with noxious substances.
   ▪ hazardous [usually before noun] hazardous substances are likely to harm people, animals, or the environment if they are not dealt with or got rid of carefully: Lead is one of the most hazardous substances known, causing cancer and nerve damage. | The building is now unoccupied, and all radioactive or hazardous materials have been removed. | hazardous nuclear waste
   ▪ deadly extremely poisonous and likely to kill you – used especially about poisons and snakes: a deadly poison | a deadly snake | Terrorists plotted to release a deadly gas.

II

deadly² adverbdeadly serious/dull/boring etc very serious, dull etc:
    I’m deadly serious, this isn’t a game!