deadly

UK: /ˈdɛdli/ US: /ˈdɛdli/ ,(ded′lē)

Inflections of 'deadly' (adj):

deadlier

adj comparative

deadliest

adj superlative

WordReference Random House Learner's Dictionary of American English © 2026

dead•ly /ˈdɛdli/   adj., -li•er, -li•est, adv. 
adj. 

1. causing or tending to cause death;
   lethal:
   a deadly disease.
2. aiming to kill or destroy:
   convicted of using a deadly weapon.
3. like death;
   deathly:
   a deadly appearance.
4. extremely boring:
   another deadly lecture.
5. excessive;
   extreme;
   very great:
   a deadly silence.
6. extremely accurate:
   a deadly shot.

adv. 

7. in a manner suggesting death:
   deadly pale.
8. excessively;
   completely:
   deadly dull.

dead•li•ness, n. [ noncount]

WordReference Random House Unabridged Dictionary of American English © 2026

dead•ly  (ded′lē), adj., -li•er, -li•est, adv. 
adj. 

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.
5. excessive;
   inordinate:
   deadly haste.
6. extremely accurate:
   Annie Oakley was a deadly shot.

adv. 

7. in a manner resembling or suggesting death:
   deadly pale.
8. excessively;
   completely:
   deadly dull.

• Middle English deedli(ch), Old English dēadlīce. See dead, -ly bef. 900

dead′li•ness, n. 
Synonyms

• 1. See fatal. 
• 4. dull, tedious, tiresome.

Collins Concise English Dictionary © HarperCollins Publishers::

deadly / ˈdɛdlɪ/ adjective ( -lier, -liest)

1. likely to cause death: deadly poison, deadly combat
2. extremely boring

adverb , adjective

1. like death in appearance or certainty: deadly pale, a deadly sleep