unconceivable

un- +‎ conceivable

unconceivable (comparative more unconceivable, superlative most unconceivable)

1. That cannot be conceived; unimaginable; inconceivable.
   • 1880, Charles Dickens, The Letters of Charles Dickens‎:

     I am now going out in a sleigh (and four) with unconceivable dignity and grandeur; mentioning which reminds me that I am informed by trusty scouts that —— intends to waylay me at Washington, and may even descend upon me in the train to-morrow.

   • 1903, Jonathan Swift, The Prose Works of Jonathan Swift, Vol. VI; The Drapier's Letters‎:

     The kind liberty granted us of wearing Indian stuffs, and calicoes, to gratify the vanity and folly of our women; which, beside the profit to England, is an unconceivable loss to us; forcing the weavers to beg in our streets, or transport themselves to foreign countries.