English Reference
adjective · adverb

deadly

adj. · adv.
US /ˈdɛdɫi/ UK /dˈɛdli/ dead·ly

  1. 01 B2 Upper Intermediate

    adj.able to cause death.

    adj.capable of causing death; lethal or fatal in effect.

    Simple

    The cobra has a deadly bite that can kill a person in hours.

    In context

    The researchers discovered that even a minute dose of the toxin proved deadly to most mammals within the ecosystem.

  2. 02 B2 Upper Intermediate

    adj.very accurate and effective, especially when aiming a weapon or a ball.

    adj.highly accurate or effective; unerring in aim or execution.

    Simple

    The striker has a deadly shot that rarely misses the goal.

    In context

    His reputation as a deadly marksman ensured that few dared to challenge him in the regional competition.

    accurateunerringprecise
  3. 03 C1 Advanced Informal

    adj.extremely dull or uninteresting.

    adj.extremely dull, tedious, or monotonous. Informal in register.

    Simple

    The three-hour lecture on tax law was absolutely deadly.

    In context

    The meeting was so deadly that several board members were visibly struggling to remain awake during the financial presentation.

  4. 04 C2 Proficiency InformalDialect

    adj.very good or cool.

    adj.excellent, impressive, or highly desirable. Primarily used in Irish and Newfoundland dialects.

    Simple

    That new jacket you bought is deadly.

    In context

    The local band played a deadly set at the pub last night, drawing a massive crowd from the surrounding villages.

    Teacher's tip

    This sense is specific to certain regions like Ireland; learners should be aware it may not be understood in the US or UK.

  5. 05 B2 Upper Intermediate

    adv.used to emphasize an adjective, meaning 'very' or 'completely'.

    adv.used as an intensifier meaning extremely or utterly; often collocates with 'serious', 'dull', or 'quiet'.

    Simple

    The room went deadly quiet when the principal walked in.

    In context

    The CEO was deadly serious when she announced that the company would be restructuring its entire European division.

    Usage

    Commonly used in the fixed phrase 'deadly serious'.

Usage deadly attacks · deadly earthquake · deadly weapon

An Australian woman, Erin Patterson, was found guilty of murdering three of her husband’s relatives and trying to kill a fourth by serving them deadly mushrooms.

The New York Times· 18 Aug 2026

Deivi Delfín and his brother moved to Colombia after a quake in Venezuela killed their mother, but were caught in another deadly disaster.

BBC News· 14 Aug 2026

From Middle English dedly, dedlych, dedlich, from Old English dēadlīċ (adjective), from Proto-West Germanic dauþalīk, from Proto-Germanic dauþalīkaz (“deadly”, literally “deathly”). By surface analysis, dead + -ly. Cognate with Saterland Frisian dodelk (“deadly”), West Frisian deadlik (“deadly”), Dutch dodelijk (“deadly”), German tödlich (“deadly”), Swedish dödlig (“deadly, fatal, mortal”), Icelandic dauðlegur (“mortal”). The adverb is from Middle English dedliche, from Old English dēadlīċe (adverb), from the adjective.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

This entry
Origin Old English dēadlīċe
Level B2 · Upper Intermediate
Frequency Common
Updated