Yours, Etc.: Origins and Uses of 8 Sign-Offs
1580s, from Late Latin lethalis, alteration of Latin letalis "deadly, fatal," from letum "death," of uncertain origin. Form altered in Late Latin by association with lethe hydor "water of oblivion" in Hades in Greek mythology, from Greek lethe "forgetfulness."
lethal le·thal (lē'thəl)
Capable of causing death.
Of, relating to, or causing death.