Definition of "unintelligible"

And indeed, my lord, there are so many passages in your writing in this controversy with me, which for their construction, as well as otherwise, are so unintelligible to me, that if I should be so unmannerly as to measure your understanding by mine, I should not know what to think of them.

1697, John Locke, Letter to the Bishop of Worcester (second reply)

The curious feature of the art was that "horse-whisperers" actually talked to the animal, usually from mouth to ear, in an unintelligible tongue; which, however, the horses appeared to understand.

1936, Rollo Ahmed, The Black Art, London: Long, page 216