Impossible
Part of speech: adjective
Definition: Not possible, not able to be done Of a person, very difficult to deal with.
Part of speech: noun
Definition: An impossibility. (with definite article) That which seems impossible.
Example sentence: Anyone who knows anything of history knows that great social changes are impossible without feminine upheaval. Social progress can be measured exactly by the social position of the fair sex, the ugly ones included.
Unthinkable
Part of speech: adjective
Definition: incapable of being believed; incredibleinconceivable or unimaginable; extremely improbable in a way that goes against common sense
Example sentence: Telling people not to have children is unthinkable and inhumane.