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

Texan communities are dealing with the impact of the deadly flash floods along the Guadalupe River, which have killed at least 80 people so far. —Rebecca Schneid, Time, 7 July 2025

Yolo County authorities announced Saturday the discovery of seven bodies at the site of Tuesday’s deadly fireworks explosion in rural Yolo County, matching the number of individuals reported missing in the blast. —Michael McGough 5, Sacbee.com, 6 July 2025

Adverb

Durst's oldest daughter was there when the shooting happened and is part of a growing tally of traumatized children left mourning a parent as Indianapolis grapples with how to curb arguments turning deadly. —Jade Jackson, IndyStar, 4 July 2025

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

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.