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

From 2021 through last season, the Knicks boasted an unfathomable 18-2 record against their Eastern Conference counterpart Washington Wizards. — Kristian Winfield, New York Daily News, 9 Oct. 2026

The Street loves this stock for some unfathomable reason, even as the company is antithetical to its aims. — Jim Cramer, CNBC, 9 Oct. 2026

With an unfathomable sense of awareness in everything from feel on the field to relationships, part of what makes him as special off the field as on. — Kansas City Star, 9 Oct. 2026

And given Lambert’s own experiences, might even seem unfathomable. — Katie McCurdy, New York Times, 2 Oct. 2026

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