Left-field and Unheard-of

Unheard-of and left-field are semantically related. In some cases you can use "Unheard-of" instead an adjective phrase "Left-field".

Unheard-of adjective - Not known or experienced before.

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