of, pertaining to, or causing death; deadly; fatal: a lethal weapon; a lethal dose.
made to cause death: a lethal chamber; a lethal attack.
causing great harm or destruction: The disclosures were lethal to his candidacy.

1575–85; < Latin lētālis, equivalent to lēt(um) death + -ālis -al1; spelling (hence pronunciation) with -h- by association with Greek lḗthē oblivion

lethality, lethalness, noun
lethally, adverb
hyperlethal, adjective
nonlethal, adjective
nonlethally, adverb
semilethal, adjective
unlethal, adjective
unlethally, adverb

1. See fatal.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
lethal (ˈliːθəl)
1.  able to cause or causing death
2.  of or suggestive of death
[C16: from Latin lēthālis, from lētum death]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

1580s, from L.L. lethalis, from L. letalis "deadly, fatal," from letum "death," of uncertain origin. Form altered in L.L. by association with lethe hydor "water of oblivion" in Hades in Greek mythology, from Gk. lethe "forgetfulness."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

lethal le·thal (lē'thəl)

  1. Capable of causing death.

  2. Of, relating to, or causing death.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
Protection against nuclear bombs, deadly chemicals, and lethal biological
  agents carries a high price.
Snake charmers defang their animals to avoid lethal bites while still giving
  the impression they are handling deadly snakes.
Taking shortcuts on patient care is a potentially lethal pursuit.
Topics include how debates over lethal injection affect the wider movement for
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