Source : Webster's Revised Unabridged Dictionary (1913)

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.

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.

Source : WordNet®

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