unintelligible
/ˌʌnɪnˈtɛlɪd͡ʒɪbəl/ adjective C2- Not intelligible; unable to be understood.
From the library
To this dumb language, which was so unintelligible to others, she answered by throwing her whole soul into the expression of her countenance, and in this manner were the conversations sustained between the blooming girl and the helpless invalid, whose body could scarcely be called a living one, but who, nevertheless, possessed a fund of knowledge and penetration, united with a will as powerful as ever although clogged by a body rendered utterly incapable of obeying its impulses.
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