Webster's Revised Unabridged Dictionary · 1913

Inconceivable

Inconceivable , adjective

[Prefix in- not + conceivable: compare French inconcevable.]

Not conceivable; incapable of being conceived by the mind; not explicable by the human intellect, or by any known principles or agencies; incomprehensible; as, it is inconceivable to us how the will acts in producing muscular motion.
It is inconceivable to me that a spiritual substance should represent an extended figure. — Locke
The inconceivableness of a quality existing without any subject to possess it. — A. Tucker