nepenthe (n.)

1590s, earlier nepenthes (1570s), "a drug or magic potion of Egypt mentioned in the 'Odyssey' as capable of banishing grief or trouble from the mind," from Greek nēpenthēs, from nē- "no, not" (from PIE root *ne- "not") + penthos "pain, grief," from PIE root *kwent(h)- "to suffer." The -s is a proper part of the word, but likely was mistaken in English as a plural affix and dropped. In medical use, "a drug having sedative properties" (1680s).