verb

fathom

to understand, to comprehend, to measure the depth of

I can't fathom why he would do that.

I can't understand why he would do that.

They tried to fathom the depths of the ocean.

They tried to measure the depth of the ocean.

((sth.)) to understand or measure (sth.) I can't fathom his reasons.

Synonyms: understand, comprehend, grasp

From Old English 'fæðm' (outstretched arms, a unit of depth). The verb sense 'to understand' evolved from the idea of measuring depth.

Imagine trying to measure a deep concept with your arms ('fathom'). If you can't reach the bottom, you can't 'fathom' it.

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