A fathom of water is 6 feet deep. Originally the term came from the Anglo-Saxon word "faetm," which means "to embrace." Back then, measurements were calculated using the average size of parts of the body, so a fathom was the distance between a man's outstretched arms from fingertip to fingertip.
Since a man would outstretch his arms to embrace his loved ones upon arriving home, it became a common measurement. Britain's Parliament declared the fathom to be an official unit of measurement.
The word "fathom" can also be used as a verb, describing an attempt to figure out a difficult problem or understand a particularly perplexing person.