First-class and Unheard-of

First-class adjective - Of the very best kind.

Unheard-of and first-class are semantically related. In some cases you can use "Unheard-of" instead an adjective phrase "First-class".

Unheard-of adjective - Not known or experienced before.

First-class and unheard-of are semantically related. Sometimes you can use "First-class" instead an adjective phrase "Unheard-of".