Unguessable

verb (used with object)
1.
to arrive at or commit oneself to an opinion about (something) without having sufficient evidence to support the opinion fully:
to guess a person’s weight.
2.
to estimate or conjecture about correctly:
to guess what a word means.
3.
to think, believe, or suppose:
I guess I can get there in time.
verb (used without object)
4.
to form an estimate or conjecture (often followed by at or about):
We guessed at the weight of the package.
5.
to estimate or conjecture correctly.
noun
6.
an opinion that one reaches or to which one commits oneself on the basis of probability alone or in the absence of any evidence whatever.
7.
the act of forming such an opinion:
to take a guess at someone’s weight.
Idioms
8.
by guess and by gosh, Northern U.S. using a combination of guesswork and reliance on luck; hit or miss.
Also, by guess and by golly.
verb (when transitive, may take a clause as object)
1.
when intr, often foll by at or about. to form or express an uncertain estimate or conclusion (about something), based on insufficient information: guess what we’re having for dinner
2.
to arrive at a correct estimate of (something) by guessing: he guessed my age
3.
(informal, mainly US & Canadian) to believe, think, or suppose (something): I guess I’ll go now
4.
keep a person guessing, to let a person remain in a state of uncertainty
noun
5.
an estimate or conclusion arrived at by guessing: a bad guess
6.
the act of guessing
7.
anyone’s guess, something difficult to predict