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]