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

That might seem almost unfathomable to anyone familiar with the last few years of FSU football, but there's still the lingering questions of Norvell's buyout now coupled with a new AD trying to get his footing in an absolute mess of a situation. — Austin Perry Outkick, FOXNews.com, 29 Sep. 2026

The masked suspect who fatally shot a beloved barber while trying to steal his handmade Miami Cuban bracelet outside a Queens strip club ran off empty-handed, but left those who knew the victim with an unfathomable hole in their hearts. — Rebecca White, New York Daily News, 25 Sep. 2026

A lot like his unfathomable eternity with the Royals, a job that he somehow was offered as a 25-year-old in 1969 with scant experience. — Kansas City Star, 24 Sep. 2026

Hasan deftly handles political seriousness and an unfathomable history and present of war and punitive state violence against Palestinians. — Matthew Carey, Deadline, 21 Sep. 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