A fathom is a unit of measurement which most often appears in the context of nautical depths. Like many seemingly odd units of measurement, the fathom was originally linked to a measurement on the human body, in this case the outstretched arms of an adult male. When measurements were standardized, the fathom as a unit of measurement was retained, and many sailors continued to use it to discuss the depth of water. As a result, many marine instruments give depths in fathoms as well as meters and feet.
Officially, a fathom measures six feet (1.8 meters) in length, although some countries quibble on fractional distances of the measurement. The fathom is an ancient unit of measurement, dating back at least to the times of Ancient Greece. The use of the old English term faethm for “outstretched arms” to discuss the measurement appears to be quite old, with the adoption of “fathom” for taking nautical soundings occurring in the 1600s. Typically, a knot would be made at each fathom length of rope, allowing sailors to count off the fathoms as they dropped the rope to the bottom.
Since one can speak of “fathoming” something in the sense of taking soundings or measurements, the term came to be used more generally to describe understanding or comprehending something. This usage of the word emerged in the early 1600s, shortly after the word began to be employed in the context of measuring depth. Something which cannot be measured or understood may be referred to as “fathomless.”
Fans of William Shakespeare may be familiar with the phrase “full fathom five thy father lies,” which appears in his play The Tempest. By convention, a burial at sea traditionally took place at a depth of at least five fathoms. It has been suggested that bodies buried at this depth are less likely to float back up, but more probably sailors simply wanted to ensure that their fellows were soundly submerged as a token of respect. On land, a traditional burial is usually performed at a depth of one fathom, leading to the slang term “six feet under.”
At one point, the fathom was also used for measuring distances on land. This trend has declined, as the fathom has been supplanted by yards, meters, feet, and other units of measurement which are smaller and therefore potentially more accurate. Old maps, however, may still indicate measurements in fathoms.