Too obscure or complicated to be understood.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "fathomless" in a sentence
"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."
"They liked to believe that King Minos of Crete, their mythical enemy from some even older past, had hidden a hellish monster that devoured the flower of Greek youth in the fathomless Labyrinth set below the palace of Knossos."
"Adrian Wilson couldn't quite bring himself to acknowledge the depths of fathomless futility."