UNTHINKABLE (adjective) definition and synonyms | Macmillan Dictionary.
What type of word is unthinkable?
inconceivable; unimaginable: the unthinkable size of the universe. not to be considered; out of the question: Such a suggestion is unthinkable.
Is Unthinkable an adverb?
unthinkable adjective - Definition, pictures, pronunciation and usage notes | Oxford Advanced Learner's Dictionary at OxfordLearnersDictionaries.com.