lethal
deadly
Today this word meansCausing death.
The source is letalis, from Latin letum (death). The original spelling had no h.
The widely accepted account is that later scholars, thinking of Lethe (lēthē), the river of forgetting, slipped the h in — and lethal was born.
Worth noting
In meaning it has nothing to do with Lethe. A misunderstanding rewrote the spelling; the word does not come from the river of the dead.