le·thal 
(lēthəl)
(lēthəl)adj.
1. Causing or capable of causing death: a lethal weapon. See Synonyms at fatal.
2. Extremely harmful; devastating: accusations lethal to the candidate's image.
[Late Latin lēthālis, alteration (probably influenced by Lēthē, Lethe) of Latin lētālis, from lētum, death.]
le·thali·ty (lē-thălĭ-tē) n.
lethal·ly adv.