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

Adjective

• lethal
• fatal

Adverb

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

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

Vietnam is prone to often deadly storms and flooding that cause widespread property damage, especially during the storm season from June until October. —Reuters, NBC news, 31 Oct. 2025

Meanwhile, Merck’s newer drug Winrevair, which is used to treat a rare, deadly lung condition, recorded $360 million in sales for the quarter. —Annika Kim Constantino, CNBC, 30 Oct. 2025

Adverb

The path this administration is on, however, is deadly serious, and the number of links in the accident chain grows daily. —Keith Wilson, MSNBC Newsweek, 18 Oct. 2025

Never before had the actor been such a formidable onscreen presence — deadly serious but still loose and limber. —Tim Grierson, Vulture, 18 Oct. 2025

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.