Difficult or impossible to understand; incomprehensible: unfathomable theories. (adjective)
Difficult or impossible to measure: the unfathomable depths. (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 "unfathomable" in a sentence
"When he looked up, his wife was staring down at him, her expression unfathomable in the darkness."
"He just looked at her, his expression unfathomable."
"Ferris had never ceased to wonder at what he called the unfathomable innocence of his wife, and he liked to go over all the points of their former life in Venice, and bring home to himself the utter simplicity of her girlish ideas, motives, and designs, which both confounded and delighted him."