unguessable

un- +‎ guessable

unguessable (comparative more unguessable, superlative most unguessable)

1. Not capable of being guessed.
   • 1828, Mary Russell Mitford, “The Fisherman in his Married State” in Our Village, London: G.B. Whittaker, p. 278,

     Never was even washerwoman more untidy. A cap all rags, from which the hair came straggling in elf-locks over a face which generally looked red-hot, surmounted by an old bonnet, originally black, now rusty, and so twisted into crooks and bends that its pristine shape was unguessable […]

   • 1898, Henry James, The Turn of the Screw, in The Two Magics, London: Heinemann, Chapter 13, p. 101,

     What it was most impossible to get rid of was the cruel idea that, whatever I had seen, Miles and Flora saw more—things terrible and unguessable and that sprang from dreadful passages of intercourse in the past.

   • 1965, Muriel Spark, The Mandelbaum Gate, London: Macmillan, Part One, Chapter 1,

     A woman of unguessable age, wearing lots of black clothes, snatched the child away […]

   • 2001, Simson Garfinkel, Gene Spafford, Web Security, Privacy and Commerce

     Especially bad are "magic words" from computer games, such as xyzzy. These magic words look secret and unguessable, but in fact they are widely known.