Which Is Not Understandable?

The most apt word I suggest is incomprehensible, but with unintelligible as a close second. Yep, incomprehensible is the best word, it means 'not understandable' but is more concise and less awkward.

Which word means not understandable?

illegible, incomprehensible, impenetrable, opaque, meaningless, unfathomable, indecipherable, vague, ambiguous, equivocal, inarticulate, incoherent, indistinct, jumbled, muddled, obscure, tenebrous, uncertain, unclear, unreadable.

What does not understandable mean?

: impossible to understand : not understandable Karl Jaspers held, famously, that schizophrenic phenomena are ununderstandable.—