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 what feels most haunting about the arc of then to now, about Smith’s unfathomable issues and woes — yet obvious heart and particularly evident quest to find himself in recent years — is that his words then were spoken with a sense of arrival and past tense. —Kansas City Star, 20 June 2026

The lack of a competitive instinct from any of the cable carriers is unfathomable, very similar to Smith Corona Marchant, which filed for reorganization in 1995. —Jim Cramer, CNBC, 14 June 2026

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

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