Source : Webster's Revised Unabridged Dictionary (1913)

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.