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

But with trillion-dollar valuations, AI developments outpacing regulators, and products that control the future flow of information, that reality isn’t all that unfathomable. —Rachyl Jones, semafor.com, 4 Mar. 2026

The brightest spots are painted by Peter Diamandis, a technology zealot who makes the case for AI infusing humanity with once-unfathomable superpowers. —ABC News, 3 Mar. 2026

Skinner allowed a goal in the shootout to Trocheck, which is all that was required to beat the Penguins, who are now an unfathomable 1-8 in shootouts. —Josh Yohe, New York Times, 28 Feb. 2026

The beads are increasingly viewed as a problem, but a Mardi Gras without beads also seems unfathomable. —Travis Loller, Arkansas Online, 16 Feb. 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