deadly

1 of 2

adjective

dead·​ly ˈded-lē 

deadlier; deadliest

1

: likely to cause or capable of producing death

deadly poison

2

a

: aiming to kill or destroy : implacable

a deadly enemy

b

: highly effective

a deadly exposé

c

: unerring

a deadly marksman

d

: marked by determination or extreme seriousness

3

a

: tending to deprive of force or vitality

a deadly habit

b

: suggestive of death especially in dullness or lack of animation

deadly bores

a deadly conversation

4

: very great : extreme

deadliness noun

deadly

2 of 2

adverb

1

archaic : in a manner to cause death : mortally

2

: suggesting death

3

: extremely

deadly serious

Synonyms of deadly

Adjective

• lethal
• fatal

Adverb

• extremely
• incredibly
• very
• terribly
• highly
• damned
• damn
• too
• badly
• so
• severely
• super
• really
• seriously
• desperately
• jolly
• intensely
• far
• most
• that

Choose the Right Synonym for deadly

deadly, mortal, fatal, lethal mean causing or capable of causing death.

deadly applies to an established or very likely cause of death.

a deadly disease

mortal implies that death has occurred or is inevitable.

a mortal wound

fatal stresses the inevitability of what has in fact resulted in death or destruction.

fatal consequences

lethal applies to something that is bound to cause death or exists for the destruction of life.

lethal gas

Examples of deadly in a Sentence

Adjective

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.

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.

Adverb

I'm deadly serious about making an offer on the house.

Recent Examples on the Web

Adjective

That extraordinary footage, along with video shot by bystanders and a 3D model created by CNN of the confrontation, shows Good and her wife challenging ICE officers – but also raises new questions about Ross’s tactics and decision to use deadly force. —Allison Gordon, CNN Money, 10 Jan. 2026

According to Homeland Security's policy, deadly force cannot be used to stop someone who is fleeing, though it is authorized when an officer believes that someone who's trying to escape poses a serious threat to the officer or others. —Jeff Wagner, CBS News, 10 Jan. 2026

Adverb

That sense of place goes a long way in immersing you into J1’s world, whether the tone is deadly serious or lightly comical. —Alphonse Pierre, Pitchfork, 9 Jan. 2026

Mostly, though, Boland strangles batters with his suffocating and deadly-accurate line and length. —Tim Spiers, New York Times, 7 Jan. 2026

Word History

First Known Use

Adjective

before the 12th century, in the meaning defined at sense 1

Adverb

before the 12th century, in the meaning defined at sense 1

Phrases Containing deadly

• deadly nightshade
• deadly sin

Rhymes for deadly

• medley

Kids Definition

deadly

1 of 2 adjective

dead·​ly ˈded-lē 

deadlier; deadliest

1

: likely to cause or capable of causing death

a deadly weapon

2

a

: aiming to kill or destroy

a deadly enemy

b

: very accurate : unerring

a deadly marksman

3

: very great : extreme

a deadly bore

deadliness noun

deadly

2 of 2 adverb

1

: suggesting death

deadly pale

2

: to a very great degree

deadly dull

Medical Definition

deadly

adjective

dead·​ly ˈded-lē 

deadlier; deadliest

: likely to cause or capable of causing death

a deadly disease

a deadly poison

a deadly instrument

deadliness

-nəs 

noun

Legal Definition

deadly

adjective

dead·​ly

deadlier; deadliest

: likely to cause or capable of causing death

also : dangerous sense 2

Note: Deadly and dangerous are sometimes used interchangeably, especially in connection with weapons or instruments.