deadly

deadly — adjective

1. able to cause the death of a living person, animal, or plant — for example, a de

1.adjectiveB1
Definition

able to cause the death of a living person, animal, or plant — for example, a deadly poison, a deadly disease, or a deadly weapon.

Example

Ryo's doctor warned him the infection could become deadly without immediate treatment.

collocation: deadly + illness (infection, disease, wound)

The young hiker survived a deadly snake bite only because she had the correct antidote in her bag.

collocation: deadly snake bite

Common collocations
  • deadly poison
  • deadly disease
  • deadly weapon
  • deadly virus
  • deadly infection
  • deadly snake
Synonyms
  • fatal

    emphasizes the certain outcome — something that actually causes death, rather than just being capable of it

  • lethal

    focuses on inherent capacity or design to kill; more formal

  • mortal

    formal and rarer; used mainly for wounds, enemies, or struggles that involve death

  • poisonous

    specific to substances that cause death or illness when absorbed by the body

Antonyms
  • harmless

    unable to cause injury or death

  • safe

    free from danger or risk of harm

Usage Note

Frequently used for poisons, diseases, weapons, dangerous animals, and hazardous situations. Can be used both before a noun (attributive: a deadly poison) and after a linking verb (predicative: this poison is deadly).

Common Mistakes

The snake is dead, so do not go near it.
The snake is deadly, so do not go near it.
'dead' means not alive; 'deadly' means able to kill you.

2. to the highest possible degree; total or absolute — used with nouns that describ

2.adjectiveB2
Definition

to the highest possible degree; total or absolute — used with nouns that describe serious qualities or states, such as seriousness, silence, or concentration.

Example

When Maja said she was moving to Tokyo alone, she had a deadly serious expression on her face.

collocation: deadly serious (fixed intensifier pair)

The courtroom fell into deadly silence as the judge began reading the guilty verdict slowly.

collocation: deadly silence

Common collocations
  • deadly serious
  • deadly silence
  • deadly concentration
  • deadly rivals
  • deadly certainty
Synonyms
  • complete

    less dramatic; simply means total with no sense of danger

  • absolute

    similar to 'complete' but slightly more formal

  • utter

    stronger than 'complete', often used with negative qualities (utter silence)

  • extreme

    broader; works in more contexts than 'deadly'

Antonyms

Usage Note

Typically used with abstract nouns describing qualities or states (seriousness, silence, concentration, rivals, certainty, precision). Does NOT apply to concrete physical objects — for example, you cannot say 'a deadly house' in this sense.

Common Mistakes

I have a deadly headache today.
I have an extremely bad headache today.
'deadly' in this sense does not describe physical pain; use it for qualities like seriousness or concentration.

3. extremely uninteresting, in a way that feels lifeless and difficult to endure —

3.adjectiveB2
Definition

extremely uninteresting, in a way that feels lifeless and difficult to endure — for example, a deadly lecture, a deadly conversation, or a deadly party.

Example

The lecture on tax law was so deadly that half the class fell asleep within the first twenty minutes.

pattern: so + deadly + that-clause

Caio called his office job deadly dull — he stared at the clock all day.

collocation: deadly dull (fixed intensifier pair)

Common collocations
  • deadly dull
  • deadly lecture
  • deadly conversation
  • deadly party
  • deadly meeting
Synonyms
  • dull

    the core meaning; less dramatic than 'deadly'

  • tedious

    slow and long, often because something is repetitive

  • monotonous

    boring because of a lack of change or variety

  • mind-numbing

    informal; so boring it feels like your brain is shutting down

Antonyms
  • exciting

    creating strong positive interest

  • thrilling

    very exciting, causing strong emotion

Usage Note

Commonly used in informal speech and writing. Often pairs with 'dull' to form the fixed phrase 'deadly dull'. This sense is always subjective — it describes a person's personal feeling of boredom, not an objective quality of the thing itself.

Common Mistakes

The film was so deadly that nobody wanted to watch it.' (correct but very informal)
The film was so boring that nobody wanted to watch it.
'deadly' in this sense is strongly informal; use 'boring' in formal or academic writing.

4. so highly effective, skilful, or powerful that it defeats any opposition or defe

4.adjectiveC1
Definition

so highly effective, skilful, or powerful that it defeats any opposition or defence — for example, deadly aim, a deadly argument, or deadly jokes.

Example

The archer drew her bow and released a deadly arrow that flew straight into the centre of the target.

collocation: deadly + weapon (arrow, aim, blow)

Quan's deadly aim with a basketball made him the highest scorer on his team throughout the entire season.

Common collocations
  • deadly aim
  • deadly accuracy
  • deadly argument
  • deadly wit
  • deadly logic
Synonyms
  • devastating

    very close in meaning; emphasizes the effect of overwhelming the opponent

  • unerring

    always accurate, never missing the target; more formal

  • lethal

    informal use meaning 'extremely good' — 'the band played a lethal set'

Antonyms
  • ineffective

    not producing the desired result

  • weak

    lacking power or skill to defeat opposition

Usage Note

Common with nouns describing skills or weapons (aim, accuracy, argument, wit, joke, logic, precision). The core idea is 'so good that nothing can stop it or defend against it'. Distinguish from sense 1 (DEATH-CAUSING): a 'deadly argument' does not literally kill anyone — it defeats an opponent's position completely.

Common Mistakes

The chef's deadly cooking killed the customers.
The chef's deadly cooking impressed every customer at the competition.
For sense 4, 'deadly' means extremely effective, not causing literal death.

deadly — adverb

Word family

  • dead
    adjectiveA1
  • die
    verbA2
  • death
    nounB1
  • deathly
    adjectiveC1
  • deaden
    verbC1

Synonym discussion

Sense: adjective/1

Deadly is the broadest of these four words. It describes anything capable of causing death — a disease, a weapon, a poison, or even a situation. Fatal emphasizes the outcome: something that actually does or will cause death. A wound can be fatal even if the weapon that caused it was not especially deadly. Lethal focuses on inherent capacity or design — a lethal dose of a substance is one that has the power to kill, whether or not it actually does. Mortal is rarer and more formal, typically used for wounds, enemies, or struggles that end in death. Example: "The snake's venom is deadly to humans, but the boy survived because doctors worked fast — a fatal outcome was avoided. A lethal injection of the same poison would have killed him for certain."

deadly ↗fatal ↗lethal ↗mortal

Common collocations

Phrases

  • deadly serious
  • deadly dull
  • deadly silence

Prepositions

Etymology

Old EnglishMiddle EnglishModern English

Deadly comes from Old English 'dēadlic', which originally meant 'subject to death' (mortal). It was formed by joining 'dēad' (dead) with the suffix '-lic' (later '-ly', meaning 'having the quality of'). By the Middle English period (around 1200–1400), the meaning had shifted to 'causing death'. Over the following centuries, speakers extended the word to describe things that were so extreme they metaphorically felt like death (deadly seriousness), and later to things so boring they felt lifeless (deadly dull).