Know how to use "fewer" and "less"? Find out.
1570s, nepenthes, from Greek nepenthes, from ne- "no, not" (see un-) + penthos "pain, grief," from PIE *kwent(h)- "to suffer" (see pathos). 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.