unheard of

adjective

1. that was never heard of; unknown: an unheard-of artist.
2. such as was never known before; unprecedented: an unheard-of salary.
3. shocking or outrageous: unheard-of conduct.

adjective

1. previously unknownan unheard-of actress
2. without precedentan unheard-of treatment
3. highly offensiveunheard-of behaviour

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]