“Open a dictionary at random; metaphors fill every page. Take the word "fathom." for example. The meaning is clear. A fathom is a measurement of water depth, equivalent to about six feet. But fathom also means "to understand." Why?

Scrabble around in the word's etymological roots. "Fathom comes from the Anglo-Saxon faethm, meaning "the two arms outstretched." The term was originally used as a measurement of cloth, because the distance from fingertip to fingertip for the average man with his arms outsretched is roughly six feet. This technique was later extended to sounding the depths of bodies of water, since it was easy to lower a cord divided into six-foot increments, or fathoms, over the side of a boat. But how did fathom come to mean "to understand," as in "I can't fathom that" or "She's unfathomable"? Metaphorically, of course.

You master something- you learn to control or accept it-when you embrace it, when you get your arms around it, when you take it in hand. You comprehend something when you grasp it, take its measure, get to the bottom of it-fathom it.

Fathom took on its present significance in classic Aristotelian fashion: through the metaphorical transfer of its original meaning (a measurement of cloth or water) to an abstract concept (understanding). This is the primary purpose of metaphor: to carry over existing names or descriptions to things that are either so new that they haven't yet been named or so abstract that they cannot be otherwise explained.”