To fathom something is to understand it thoroughly. It's usually used in the negative, as in "I can't fathom why he doesn't want to go along with us."

Fathom is from Middle English fadme, from Old English fæthm, "outstretched arms." The Old English and modern English noun also refers to a unit of length used to measure the depth of water. The sense of intellectual depth comes from that meaning — to understand or fathom something thoroughly is "to get to the bottom of it."

Definitions of fathom
  1. noun
    a linear unit of measurement (equal to 6 feet) for water depth
    synonyms: fthm
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    type of:
    linear measure, linear unit
    a unit of measurement of length
  2. noun
    (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore
  3. verb
    measure the depth of (a body of water) with a sounding line
    synonyms: sound
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    type of:
    measure, quantify
    express as a number or measure or quantity
  4. verb
    come to understand
    synonyms: bottom, penetrate
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    type of:
    know and comprehend the nature or meaning of
Word Family
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