adj. dead·li·er, dead·li·est
1. Causing or tending to cause death: deadly weapons; a deadly spill of radioactive waste products. See Synonyms at fatal.
2. Suggestive of death: a deadly pallor.
3. Aiming or wanting to kill; implacable: deadly enemies.
a. Destructive in effect: gave the film a deadly review.
b. Tending to take away vitality, effectiveness, or force: the deadly habit of procrastination.
5. Absolute; utter: deadly concentration.
6. Extreme or terrible: worked under deadly strain.
7. Extremely accurate; unerring: She is a deadly shot with the bow.
8. Dull, tedious, and boring: a deadly prose style.
1. So as to suggest death.
2. To an extreme: deadly serious.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.