unfathomable

Etymology

It is from English un- + fathom + -able,^[2][3] though Etymonline and the Shorter Oxford analyse it as un- and fathomable, an adjective first recorded in 1633.^[4][5][2] The verb, to measure the depth of water and later to get to the bottom of, is from Old English fæðmian (“to embrace, surround”),^[4][2] from fæðm (“the outstretched arms”), from Proto-Germanic *faþmaz.^[4][2][6]

Earliest recorded meaning

Figuratively, of feelings, qualities or conditions, that cannot be fully explored or understood: 1617, in a book by Collins.^[2] Etymonline also dates the figurative sense to the 1610s,^[4] and Merriam-Webster Unabridged gives 1640.^[3]

Semantic evolution

The verb fathom had been used of taking soundings since about 1600 and of understanding something thoroughly since the 1620s,^[4] so the figurative adjective came first.^[4][2] In 1640, in Bishop Hall's Christian Moderation, it was used of something immeasurable or vast,^[2] and the literal sense, of space or water too deep to be measured, followed in Hale's The Primitive Origination of Mankind, before 1676.^[2][4] Johnson (1755) defined it as "not to be sounded by a line", quoting Addison on a lake that the local people claimed was bottomless.^[7] From 1817, in Shelley's Laon and Cythna, it was also used of the eyes.^[2] The Shorter Oxford calls the word chiefly figurative.^[5]

Timeline

• 1617 – figuratively, that cannot be fully explored or understood.^[2][4]
• 1640 – immeasurable, vast, in figurative contexts.^[2][3]
• before 1676 – of space, too deep to be measured.^[2][4]
• 1817 – of the eyes.^[2]

Meanings

Adjective

1. Impossible to understand or explain; inscrutable: His reasons for leaving remained unfathomable to his friends.^[8][3][5]
2. Too deep to be measured; immeasurable or vast: an unfathomable gulf.^[8][3][2]

Word family

• unfathomability
• unfathomably
• unfathomableness

Related words

• fathom
• fathomless
• unfathomed

References

1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i j k l m n Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d e Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
4. ^ a b c d e f g h i Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
5. ^ a b c Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
6. ^ Kroonen, Guus (2013), Etymological Dictionary of Proto-Germanic, Leiden Indo-European Etymological Dictionary Series 11, Leiden / Boston: Brill
7. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
8. ^ a b Collins English Dictionary (2023), Glasgow: HarperCollins