unimaginable

un‧i‧ma‧gin‧a‧ble /ˌʌnɪˈmædʒənəbəl◂, ˌʌnɪˈmædʒɪnəbəl◂/ BrE AmE adjective
[Word Family: adjective: imaginable ≠ unimaginable, imaginary, imaginative ≠ unimaginative, unimagined; noun: imagination, imaginings; adverb: unimaginably, imaginatively; verb: imagine]
not possible to imagine:
unimaginable wealth
—unimaginably adverb

Oxford Advanced Learner's Dictionary

unimaginable

un·imagin·able [unimaginable unimaginably] [ˌʌnɪˈmædʒɪnəbl] [ˌʌnɪˈmædʒɪnəbl] adjective (formal)
impossible to think of or to believe exists; impossible to imagine
• unimaginable wealth
• This level of success would have been unimaginable just last year.
Opp: imaginable

Derived Word: unimaginably

Cambridge Advanced Learner's Dictionary

Cambridge Advanced Learner's Dictionary - 4th Edition

unimaginable / ˌʌn.ɪˈmædʒ.ɪ.nə.bl̩ / adjective

C2 describes something that is difficult to imagine because it is so bad, good, big, etc.:

unimaginable pain/wealth

unimaginably / -bli / adverb

Collins COBUILD Advanced Learner’s English Dictionary

unimaginable

[ʌ̱nɪmæ̱ʤɪnəb(ə)l]
ADJ-GRADED (emphasis)
If you describe something as unimaginable, you are emphasizing that it is difficult to imagine or understand properly, because it is not part of people's normal experience.
  The scale of the fighting is almost unimaginable...
  The children here have lived through unimaginable horrors.
Syn:
unbelievable
  Derived words:
  unimaginably [ʌ̱nɪmæ̱ʤɪnəbli] ADV-GRADED ADV adj Conditions in prisons out there are unimaginably bad.

Merriam-Webster's Advanced Learner's Dictionary

Merriam-Webster's Advanced Learner's Dictionary:

unimaginable

un·imag·in·able /ˌʌnəˈmæʤənəbəl/ adj [more ~; most ~] : not possible to imagine : beyond what you would normally imagine
• the unimaginable horrors of war
• a disaster of almost unimaginable proportions
• This technology would have been unimaginable five years ago.
- un·imag·in·ably /ˌʌnəˈmæʤənəbli/ adv
• unimaginably large numbers