unfathomable

adjective

un·​fath·​om·​able ˌən-ˈfa-t͟hə-mə-bəl 

: not capable of being fathomed:

a

: immeasurable

… the unfathomable darkness of empty space.

—H. G. Wells

b

: impossible to comprehend

unfathomable motives/reasons

unfathomably

ˌən-ˈfa-t͟hə-mə-blē 

adverb

Synonyms of unfathomable

• infinite
• endless

Examples of unfathomable in a Sentence

His behavior is completely unfathomable.

the unfathomable reaches of space

Recent Examples on the Web

By one of those unfathomable mysteries of airline pricing, the last-minute booking was, counterintuitively, lower than usual. —Tom Vanderbilt, Travel + Leisure, 31 Dec. 2025

Not having him as a part of me anymore felt unfathomable. —Miriam Finder Annenberg, CNN Money, 28 Dec. 2025

Kobe Bryant January 2020 had one of the darkest and most unfathomable moments in sports. —Ryan Gaydos, FOXNews.com, 22 Dec. 2025

Inequality grew, especially at the very top, where individuals amassed unfathomable wealth (hundreds of billions of dollars) and some corporate valuations hit unreal heights ($4 trillion to $5 trillion). —Andrew Winston, Fortune, 21 Dec. 2025

Word History

First Known Use

1640, in the meaning defined above

Kids Definition

unfathomable

adjective

un·​fath·​om·​able ˌən-ˈfat͟h-ə-mə-bəl 

: not capable of being fathomed:

a

: immeasurable

b

: impossible to understand