rare vs unheard-of

rare

• Very uncommon; scarce. 

• Thin; of low density. 

• Particularly of meat, especially beefsteak: cooked very lightly, so the meat is still red. 

• Good; enjoyable. 

• To rear, bring up, raise. 

• To rear, rise up, start backwards. 

• A scarce or uncommon item. 

unheard-of

• Unknown or known to only few people. 

• Previously unknown; unprecedented.