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

The 20-year-old has struck out 32 batters in 18 2/3 innings — an almost unfathomable rate. —Charlotte Varnes, New York Times, 2 June 2026

This is not like building to be able to compete with the relentless depth of the Thunder or the unfathomable length of Victor Wembanyama and youth of the Spurs. —Ira Winderman, Sun Sentinel, 30 May 2026

Somehow, the Avs found a different, yet equally unfathomable way to lose. —Corey Masisak, Denver Post, 27 May 2026

When the Hawks’ season ended last month, following an unfathomable 51-point loss to the Knicks in Game 6 of the first-round Eastern Conference series, the prevailing notion seemed to be that the Hawks were fatally flawed. —Lauren Williams, AJC.com, 23 May 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