indemonstrable
Concise Oxford English Dictionary © 2008 Oxford University Press:
indemonstrable /ˌɪndɪˈmɒnstrəb(ə)l, ɪnˈdɛmən-/
▶adjective not able to be proved or demonstrated.
■ Philosophy (of a truth) axiomatic and hence not provable.
'indemonstrable' also found in these Oxford entries:

