lethal

lethal — adjective

1. Strong enough or dangerous enough to kill a person, animal, or other living thin

1.adjectiveB2
Definition

Strong enough or dangerous enough to kill a person, animal, or other living thing — used about poisons, weapons, diseases, doses of medicine, or physical force.

Example

The venom of this snake is highly lethal — a single bite can kill an adult.

collocation: highly lethal

Police officers are trained to use lethal force only when protecting lives demands it.

collocation: lethal force

Common collocations
  • lethal dose
  • lethal injection
  • lethal force
  • lethal weapon
  • lethal poison
  • highly lethal
  • potentially lethal
  • non-lethal
Synonyms
  • deadly

    more common in everyday language; broader range including poison, disease, enemy

  • fatal

    focuses on the result (death has occurred), not the capacity

  • mortal

    more formal or literary; often used about wounds or danger

Antonyms

Grammar Patterns

lethal + noun (a lethal dose)

be + lethal

be lethal to + noun phrase

Usage Note

Everyday synonyms: 'deadly' is far more common in casual speech; 'lethal' is more formal and appears frequently in medical, legal, and military contexts. Distinguish from 'fatal': 'fatal' describes the outcome (a fatal accident), whereas 'lethal' describes the thing's capacity to kill (a lethal weapon).

Common Mistakes

The car crash was lethal.
The car crash was fatal.
'Lethal' describes the potential of a thing to cause death, not the outcome of an event. Use 'fatal' for events that actually resulted in death.

2. So harmful or damaging to plans, relationships, businesses, or reputations that

2.adjectiveC1
Definition

So harmful or damaging to plans, relationships, businesses, or reputations that they are completely ruined or destroyed.

Example

The combination of rising costs and falling sales proved lethal to the family business.

pattern: prove lethal to [abstract entity]

Obi's careless comment had a lethal effect on their friendship, and they never spoke again.

collocation: lethal effect

Common collocations
  • lethal combination
  • lethal effect
  • lethal mix
  • lethal to reputation
  • lethal to business
Synonyms
  • devastating

    emphasises the shock and emotional impact of the damage

  • destructive

    focuses on the act of breaking or ruining something

  • ruinous

    formal; suggests total financial or structural collapse

Antonyms

Grammar Patterns

be lethal to + noun phrase (abstract)

lethal + noun (abstract)

Usage Note

This meaning extends the death-related idea to abstract things — a plan, career, relationship, or reputation can be 'killed'. The preposition 'to' is standard when stating what is harmed.

Common Mistakes

The criticism was lethal for his career.
The criticism was lethal to his career.
Use 'to', not 'for', after 'lethal' in this abstract sense.

lethal — noun

Word family

  • lethally
    adverbB2
  • lethality
    nounC1

Synonym discussion

Sense: adjective/1

All three words relate to death, but their focus differs. 'Lethal' points to the inherent capacity of a thing to cause death — a weapon, poison, or dose is lethal by design or property. 'Deadly' is the most common and versatile of the three: it covers anything that can cause death (a deadly disease, a deadly enemy, a deadly mistake) and is used in both formal and informal settings. 'Fatal' looks backward at the outcome — a fatal accident is one that actually ended in death; you would not normally say 'a fatal weapon'. 'Mortal' is more formal or literary, often describing a wound or danger that is certain to end in death ('a mortal blow'). The choice depends on whether the speaker is describing capacity (lethal), result (fatal), general danger (deadly), or literary finality (mortal). For example: 'The knife was a lethal weapon' (capacity) vs. 'The stab wound proved fatal' (outcome) vs. 'He received a deadly blow to the head' (general danger).

deadly ↗fatal ↗mortal

Common collocations

Nouns

Adverbs

  • lethally
  • potentially lethal
  • highly lethal
  • almost lethal

Phrases

  • lethal dose
  • lethal injection
  • lethal force
  • lethal weapon
  • lethal combination
  • lethal mix
  • prove lethal to
  • carry a lethal

Prepositions

Etymology

LatinEnglish

The word 'lethal' comes directly from Latin 'letalis', meaning 'deadly' or 'causing death', which in turn comes from 'letum', the Latin word for 'death' or 'destruction'. It entered English in the late 1500s as a formal alternative to existing Germanic words like 'deadly'. The spelling was influenced by the related Latin noun 'lethum'. Unlike 'fatal' (from Latin 'fatum' meaning 'fate'), 'lethal' keeps its focus on the power to kill rather than on destiny or outcome.