From Middle English dedly, dedlych, dedlich, from Old English dēadlīċ.

The adverb is from Middle English dedliche, from Old English dēadlīċe, from the adjective.

• IPA: /ˈdɛd.li/

deadly (comparative deadlier, superlative deadliest)

1. (obsolete, rare) Subject to death; mortal.
2. Causing death; lethal.
   • 1879, R[ichard] J[efferies], chapter 1, in The Amateur Poacher, London: Smith, Elder, & Co., […], →OCLC ↗:

     But then I had the [massive] flintlock by me for protection. ¶ […] The linen-press and a chest on the top of it formed, however, a very good gun-carriage; and, thus mounted, aim could be taken out of the window […], and a 'bead' could be drawn upon Molly, the dairymaid, kissing the fogger behind the hedge, little dreaming that the deadly tube was levelled at them.

   • 1949 June 8, George Orwell [pseudonym; Eric Arthur Blair], chapter 9, in Nineteen Eighty-Four: A Novel, London: Secker & Warburg, →OCLC ↗; republished [Australia]: Project Gutenberg of Australia, August 2001, part 2, page 177 ↗:

     […] others search for new and deadlier gases, or for soluble poisons capable of being produced in such quantities as to destroy the vegetation of whole continents […]

3. Aiming or willing to destroy; implacable; desperately hostile.

   deadly enemies

   • c. 1601–1602 (date written), William Shakespeare, “Twelfe Night, or What You Will”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act III, scene iv], page 269 ↗, column 1:

     […] diſmount thy tucke, be yare in thy preparation, for thy aſſaylant is quick, skilfull and deadly.

4. Very accurate (of aiming with a bow, firearm, etc.).
5. (informal) Very boring.
   • 1907 August, Robert W[illiam] Chambers, chapter VI, in The Younger Set, New York, N.Y.: D. Appleton & Company, →OCLC ↗:

     “I don't mean all of your friends—only a small proportion—which, however, connects your circle with that deadly, idle, brainless bunch—the insolent chatterers at the opera, the gorged dowagers, the worn-out, passionless men, the enervated matrons of the summer capital, […]!”

6. (informal, Australian Aboriginal, Ireland, Newfoundland) Excellent, awesome, cool.

• French: mortel, fatal, létal
• German: tödlich
• Portuguese: mortal, fatal, letal, mortífero, fatífero
• Russian: смерте́льный
• Spanish: mortal, letal, mortífero

• French: mortel

• Russian: великоле́пный

deadly

1. (obsolete) Fatally, mortally.
   • 1603, Michel de Montaigne, “Our affections are tranſported beyond our ſelues”, in John Florio, transl., The Essayes […], book I, London: […] Val[entine] Simmes for Edward Blount […], →OCLC ↗, page 7 ↗:

     [P]erceiving himſelfe deadly wounded by a ſhot received in his body, being by his men perſwaded to come off and retire himſelfe from out the throng, anſwered, he would not now ſo neere his end, beginne to turne his face from his enemie […]

2. In a way which suggests death.

   Her face suddenly became deadly white.

3. Extremely, incredibly.

• Russian: смертельно

• Russian: смертельно

• dead