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Synonyms

deadly

American  
[ded-lee] / ˈdɛd li /

adjective

deadlier, deadliest

  1. causing or tending to cause death; fatal; lethal.

    a deadly poison.

  2. aiming to kill or destroy; implacable.

    a deadly enemy.

  3. like death.

    a deadly pallor.

  4. excruciatingly boring.

    The dinner party was absolutely deadly.
    Synonyms:
    dreary, tedious, dull

  5. excessive; inordinate.

    deadly haste.

  6. extremely accurate.

    Annie Oakley was a deadly shot.

adverb

  1. in a manner resembling or suggesting death.

    deadly pale.

  2. excessively; completely.

    deadly dull.
deadly British  
/ ˈdɛdlɪ /

adjective

  1. likely to cause death

    deadly poison

    deadly combat

  2. informal extremely boring

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

adverb

  1. like death in appearance or certainty

    deadly pale

    a deadly sleep
"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Related Words

See fatal.

Other Word Forms

  • deadliness noun
  • nondeadly adjective

Etymology

Origin of deadly

First recorded before 900; Middle English deedli(ch), Old English dēadlīce; dead, -ly

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

Only a Mars landing ranks lower, while monitoring for deadly asteroids and measuring climate change are top priorities among Americans.

From Salon • Apr. 7, 2026

Air India's chief executive officer Campbell Wilson has resigned as the airline deals with challenges, including financial losses and the aftermath of a deadly crash last year that killed 260 people.

From BBC • Apr. 7, 2026

Damburger survived the deadly 2018 Carr fire that took out a swath of west Redding, burning within two miles of the restaurant.

From Los Angeles Times • Apr. 7, 2026

Keenan played a similar role in the civil-rights prosecutions of Louisville, Ky., police officers involved in the deadly 2020 raid on Breonna Taylor’s home.

From The Wall Street Journal • Apr. 7, 2026

Carriage accidents were serious in wartime Washington and could prove deadly.

From "Chasing Lincoln's Killer" by James L. Swanson

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.