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

Martin’s most poignant moment all season is her inscrutable look as Raskova gazes between Anastasia and Sasha; two people sacrificing themselves for love is unfathomable to her. — Roxana Hadadi, Vulture, 10 July 2026

Google Books and the Internet Archive represent libraries of unfathomable proportions. — Rose Horowitch, The Atlantic, 8 July 2026

Much of what already happened was unfathomable when RCX Sports won the NFL FLAG contract, which has been renewed several times. — Tim Graham, New York Times, 2 July 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