Wiktionary
uncommunicable
a. Impossible to communicate.
Usage examples of "uncommunicable".
But as the War in Beleriand was at the time of the hobbits' meeting some 7,000 years ago, no doubt they were not quite the same: less wise, less strong, shyer and more uncommunicable (their own language simpler, but their knowledge of other tongues very small).
For a moment youth and age shared a knowledge uncommunicable to anyone unacquainted with that particular animal.