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

• baleful
• deathly
• fatal
• fell
• killer
• lethal
• mortal
• murderous
• pestilent
• terminal
• vital

Adverb

• achingly
• almighty
• archly
• awful
• awfully
• badly
• beastly
• blisteringly
• bone
• colossally
• corking
• cracking
• damn
• damned
• dang
• desperately
• eminently
• enormously
• especially
• ever
• exceedingly
• exceeding
• extra
• extremely
• fabulously
• fantastically
• far
• fiercely
• filthy
• frightfully
• full
• greatly
• heavily
• highly
• hugely
• immensely
• incredibly
• intensely
• jolly
• majorly
• mightily
• mighty
• monstrous [chiefly dialect]
• mortally
• most
• much
• particularly
• passing
• rattling
• real
• really
• right
• roaring
• roaringly
• seriously
• severely
• so
• sore
• sorely
• spanking
• specially
• stinking
• such
• super
• supremely
• surpassingly
• terribly
• that
• thumping
• too
• unco
• uncommonly
• vastly
• very
• vitally
• way
• whacking
• wicked
• 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

Days before Trump ordered the drone strike that killed him, Soleimani had orchestrated a deadly attack on a U.S. base in Iraq. —Michael Ruiz, FOXNews.com, 24 June 2025

Since Israel launched its Gaza offensive in the wake of Hamas' deadly October 2023 attack, the country has faced war crimes accusations from United Nations officials and human rights groups. —Micah McCartney, MSNBC Newsweek, 24 June 2025

Adverb

While Alan Wake and Control aren’t themselves deadly serious, Firebreak hues much more heavily into broad comedy than the mainline Remedyverse titles. —Christopher Cruz, Rolling Stone, 19 June 2025

Air India last suffered a crash this deadly in 2010, when 158 people were killed after one of its jets overshot a runway in southern India. —Christian Edwards, CNN Money, 12 June 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.