Wiktionary
undemonstrable
a. not demonstrable
Usage examples of "undemonstrable".
No, all this can not be drawn, can not be expressed, but remains undemonstrable, indefinable.
Gaposchkin attacked Velikovsky again in the June 1950 issue of Popular Astronomy, the essence of the argument being that his claims couldn't be true because they violated undemonstrable dogmatisms that antedated him and therefore took precedence.