lethal (adj.) Look up lethal at Dictionary.com
"causing or resulting in death," 1580s, from Late Latin lethalis, alteration of Latin letalis "deadly, fatal," from lethum/letum "death," a word of uncertain origin. The form altered in Late Latin by association with lethes hydor "water of oblivion" in Hades in Greek mythology, from Greek lethe "forgetfulness" (see Lethe).