adj. Very deep (especially of water deeper than a lead line can measure); bottomless
adj. unfathomable or incomprehensible
the GNU version of the Collaborative International Dictionary of English
adj. Incapable of being fathomed; immeasurable; that can not be sounded.
adj. Incomprehensible.
The Century Dictionary and Cyclopedia
Incapable of being embraced or encompassed with the arms.
Having a depth so great that it cannot be fathomed; bottomless.
Not to be penetrated by thought or comprehended.
Word Usage
"It is indeed a fathomless mystery of theology, and even if I were theologian enough to deal with it directly, it would not be relevant to do so here."