Units of measurement for water depth (equal to 6 feet), or to understand something after thinking about it carefully.
From Old English 'fæthm' meaning outstretched arms, since a fathom originally measured the distance between a man's outstretched fingertips. The term evolved to describe water depth because sailors measured it with rope marked by fathom lengths. The verb 'fathom out' eventually shortened to just 'fathom,' meaning to figure out or comprehend.
Sailors needed a quick way to measure water depth without instruments, so they just stretched out their arms! The average man's arm span was about 6 feet—which became the standard fathom measurement used by mariners for centuries. It's why we still say we 'cannot fathom' something when we can't understand it.
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