That cannot be ascertained; uncertain, unable to be determined or discovered. (adjective)
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Use "unascertainable" in a sentence
"In his questioning of Osterhaus, Shepherd asked whether DCF had a means to determine whether prospective adoptive parents are susceptible to mental illness, drug abuse or domestic violence -- or whether such risks were '' unascertainable. ''"
"The back yard is unfenced, and extends to the skyline and an unascertainable bit beyond."
"It's pretty preposterous, though, since -- putting aside the larger problem of turnout for Obama being suppressed by the knowledge that the election was unsanctioned -- there are presumably a fair, but unascertainable number of people who voted for Clinton who might have voted for Obama (or Edwards, or Richardson) had they been on the ballot."