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

Two months of heavy rainfall resulted in then-unfathomable devastation that still reverberates today. — Kansas City Public Library Staff, Kansas City Star, 1 July 2026

To some, the Galatasaray forward’s selection in the starting line-up is unfathomable and is now being used to portray Nagelsmann as stubborn or unwilling to react to performances. — Sebastian Stafford-Bloor, New York Times, 26 June 2026

The money the Hornets still owed Ball was almost unfathomable in normal circles, but not in the NBA’s alternate universe. — Scott Fowler, Charlotte Observer, 25 June 2026

The race to $1 trillion once seemed unfathomable, but Musk has climbed the ranks of the world’s wealthiest at a rapid pace. — Danielle Chemtob, Forbes.com, 15 June 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