Unheard-of and Whatchamacallit

Unheard-of adjective - Not known or experienced before.

Whatchamacallit and unheard-of are semantically related In some cases you can use "Whatchamacallit" instead an adjective phrase "Unheard-of".

Whatchamacallit noun - A small article the actual name of which one either does not know or cannot remember.

Unheard-of and whatchamacallit are semantically related in unknown topic. Sometimes you can use "Unheard-of" instead an adjective "Whatchamacallit".