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

In Grasso’s Garage this week, another wagon, but with unfathomable usage, power, styling, and, most importantly, comfort! —Marc D. Grasso, Boston Herald, 15 Jan. 2026

In his four years as a starter at Boise State, Moore went an unfathomable 50-3. —Shaun Goodwin, Idaho Statesman, 15 Jan. 2026

Just an unfathomable thing, given the talent available and how excellent this club has been at nearly everything else. —Corey Masisak, Denver Post, 14 Jan. 2026

Wembanyama, with an unfathomable ability to elongate into plays with three capable guards flanking him (De’Aaron Fox, Steph Castle and Dylan Harper combined for a modest 46 points Tuesday), poses a threat worthy of OKC’s critical thinking. —Joel Lorenzi, New York Times, 14 Jan. 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