The least used words in the English language are rare and often archaic:

1. Quixotic – Extremely idealistic, unrealistic, and impractical.
2. Sesquipedalian – Characterized by long words; long-winded.
3. Pulchritudinous – Physically beautiful or attractive.
4. Defenestration – The act of throwing someone out of a window.
5. Antediluvian – Extremely old-fashioned; before the Biblical flood.