verb

fathom

to understand, to comprehend, to gauge

I can't fathom why he left.

I can't fathom why he left.

It is hard to fathom the vastness of space.

It is hard to fathom the vastness of space.

((sth.)) to understand (sth.) I can't fathom his decision.

((why/how/what...)) to understand (why/how/what...) I can't fathom why he did it.

Synonyms: understand, comprehend, grasp, penetrate

From Old English 'fæðm', meaning 'outstretched arms', a unit for measuring depth. The verb sense comes from the idea of 'getting to the bottom of' something.

Imagine trying to measure the depth of the ocean with your arms (a fathom). It's so deep you can't 'fathom' it, or understand its depth.

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