1911 Encyclopædia Britannica/Lethal

LETHAL (Lat. lethalis, for letalis, deadly, from letum, death; the spelling is due to a confusion with Gr. λήθη, forgetfulness), an adjective meaning “deadly,” “fatal,” especially as applied to weapons, drugs, &c. A “lethal chamber” is a room or receptacle in which animals may be put to death painlessly, by the admission of poisonous gases.