unheard of
Extremely uncommon or unusual; beyond belief. Hyphenated if used as a modifier before a noun. His prowess on the balance beam is unheard of. Investors began making unheard-of amounts of money on these risky investments.
unheard of
Very unusual, extraordinary, as in It's unheard of to have all one's money refunded two years after the purchase. This expression alludes to a circumstance so unusual that it has never been heard of. [Late 1500s]