Is Unthinkably a Real Word?

un·think·a·ble. adj. 1. Impossible to imagine; inconceivable: "The inert gases have been coaxed into combination, and I have seen a fluoride of xenon, unthinkable in the 1940s" (Oliver Sacks).

What is unthinkable?

1 : not capable of being grasped by the mind. 2 : being contrary to what is reasonable, desirable, or probable : being out of the question.

What does Impractible mean?

1 : impassable an impracticable road. 2 : not practicable : incapable of being performed or accomplished by the means employed or at command an impracticable proposal.