Unguessable

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.

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

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.