First-rate and Unheard-of

First-rate adjective - Of the very best kind.

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

Unheard-of adjective - Not known or experienced before.

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