The Collaborative International Dictionary of English v.0.48:

Lethal \Leth"al\ (l[e^]th"[a^]l), n. [Lauric + ether + alcohol.]
   (Chem.)
   One of the higher alcohols of the paraffine series obtained
   from spermaceti as a white crystalline solid. It is so called
   because it occurs in the ethereal salt of lauric acid.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lethal \Le"thal\ (l[=e]"thal), a. [L. lethalis, letalis, fr.
   lethum, letum, death: cf. F. l['e]thal.]
   Deadly; mortal; fatal. "The lethal blow." --W. Richardson. --
   Le"thal*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lethal
    adj 1: of an instrument of certain death; "deadly poisons";
           "lethal weapon"; "a lethal injection" [syn: deadly,
           lethal]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "lethal":
   baleful, baneful, brutal, corroding, corrosive, corrupting,
   corruptive, counterproductive, damaging, deadly, death-bringing,
   deathful, deathly, deleterious, destructive, detrimental,
   disadvantageous, disserviceable, distressing, fatal, feral,
   harmful, hurtful, injurious, internecine, killing, malefic,
   malevolent, malign, malignant, mischievous, mortal, noisome,
   noxious, ominous, pernicious, pestilent, pestilential, poisonous,
   prejudicial, savage, scatheful, toxic, venenate, veneniferous,
   venenous, venomous, vicious, virulent