"As it seems to be an unguessable riddle—do," urged his sister.
It looked as if it had some unguessable but rarely-used purpose.
Galen Albret, inscrutable, grim, brooded his unguessable thoughts.
Then he was looking straight down into a milky transparency that started under his nose and continued into unguessable depths.
In the beam of the searchlight we saw that the submarine cavern extended on and on for an unguessable depth.
A child of intelligences, remote and unguessable, working corporally in metals, it indubitably was.
For some reason unknown, unguessable, the champion she fought for had fled from the fight.
For an instant, a quick, unguessable emotion clouded the wide black eyes and tightened the thin lips, then it was gone.
Vaguely, he could see people and, behind them, machines so simply designed that their functions were unguessable.
If he could come out of the dark, it was full of unguessable things, splendid and strange and new.
c.1300, gessen "to estimate, appraise," originally "take aim," probably from Scandinavian (cf. Middle Danish gitse, getze "to guess," Old Norse geta "guess, get"), possibly influenced by Middle Dutch gessen, Middle Low German gissen "to guess," all from Proto-Germanic *getiskanan "to get" (see get). Sense evolution is from "to get," to "to take aim at," to "to estimate." Meaning "to hit upon the right answer" is from 1540s. U.S. sense of "calculate, recon" is true to the oldest English meaning. Spelling with gu- is late 16c., sometimes attributed to Caxton and his early experience as a printer in Bruges. Related: Guessed; guessing. Guessing game attested from 1650s.
c.1300, from guess (v.). Verbal shrug phrase your guess is as good as mine attested from 1902.