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

So to not have caught this at some earlier stage was just unfathomable to me. — Kerry Breen, CBS News, 13 June 2026

That level of wealth, all owned by just one person, was once unfathomable. — Wyatte Grantham-Philips, Fortune, 12 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

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

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