nepenthe (n.)
1570s, nepenthes, from Greek nepenthes, from ne- "no, not" (from PIE root *ne- "not") + penthos "pain, grief," from PIE root *kwent(h)- "to suffer." A drug of Egypt mentioned in the "Odyssey" as capable of banishing grief or trouble from the mind. The -s is a proper part of the word, but likely was mistaken in English as a plural affix and dropped.