Something that's deadly can kill you. Because a gun can end someone's life, it's a deadly weapon.

If someone's diagnosed with a deadly disease, it's lethal — it's capable of ending the person's life. When you hear someone talk about police officers using "deadly force," they mean actions that intended to cause death, or understood to be capable of killing someone. You can also use the word to mean "extremely" or "very," as when you describe your math class as deadly boring.

Definitions of deadly
  1. adjective
    causing or capable of causing death
    “a deadly enemy”
    synonyms: deathly, mortal
    bringing death
  2. adjective
    of an instrument of certain death
    deadly poisons”
    synonyms: lethal
    bringing death
  3. adjective
    exceedingly harmful
    synonyms: baneful, pernicious, pestilent
    injurious to physical or mental health
  4. adjective
    (of a disease) having a rapid course and violent effect
    infectious; having the ability to cause disease
  5. adjective
    extremely poisonous or injurious; producing venom
    synonyms: venomous, virulent
    of or relating to or caused by a toxin or poison
  6. adjective
    involving loss of divine grace or spiritual death
    “the seven deadly sins”
    synonyms: mortal
    not admitting of pardon
  7. adverb
    as if dead
    synonyms: lifelessly
  8. adverb
    (used as intensives) extremely
    deadly dull”
    deadly earnest”
    synonyms: deucedly, devilishly, insanely, madly
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