/ ˈfæðəm; NAmE / verb , noun
■ verb
fathom sb/sth (out) to understand or find an explanation for sth :
[ vn ]
It is hard to fathom the pain felt at the death of a child.
[ v wh- ]
He couldn't fathom out what the man could possibly mean.
■ noun
a unit for measuring the depth of water, equal to 6 feet or 1.8 metres :
The ship sank in 20 fathoms.
( figurative )
She kept her feelings hidden fathoms deep.
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WORD ORIGIN
Old English fæthm , of Germanic origin; related to Dutch vadem , vaam and German Faden six feet. The original sense was something which embraces , (plural) the outstretched arms ; hence, a unit of measurement based on the span of the outstretched arms, later standardized to six feet.