Mike from Glen Arbor asked about the word fathom, especially when it means to understand, as in, “I saw something in that lad's eye I never quite fathomed.” [Charlotte Brontë Villette III. xxxix. 221]
As the word existed in the original Germanic/Scandinavian forms, it referred to outstretched arms, especially arms outstretched to hug someone. If you measure the outstretched arms of the average adult male from tip of longest finger to tip of longest finger, the approximate length is six feet.
The idea of hugging eventually drifted into the background, and the measurement of six feet moved front and center. That became the standard for sounding, measuring the depth of water by dropping a line knotted every six feet. Shakespeare used it that way in The Tempest: “Full fadom fiue thy Father lies.”
The idea of understanding something thoroughly arose by analogy, the idea being that you would dive into or plumb the depths of a person’s mind, and immerse yourself in his or her thoughts.
Available from McFarland & Co.: Word Parts Dictionary, 2nd edition
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