The longest of many units derived from an anatomical measurement, the fathom originated as the distance from the middle fingertip of one hand to the middle fingertip of the other hand of a large man holding his arms fully extended. The name comes from the Old English faedm or faethm, meaning outstretched arms.

Why is fathom used?

A fathom is a unit of length in the imperial and the U.S. customary systems equal to 6 feet (1.8288 m), used especially for measuring the depth of water.

When was fathom first used?

The first known use of fathom was before the 12th century.