If you’re anything like me, you’re aware that fathom is a unit of measurement (though fuzzy on the details of what it equals, perhaps) but use it most often as a synonym for “understand” or “comprehend.” Ever wonder how these meanings are related? Because they totally are.
Fathom comes all the way from Old English as both a noun and a verb, both coming from the same meaning. The verb was “to embrace or surround” and the noun was “the length of outstretched arms,” so about 5-6 feet on average.
By about 1600 it was used as a verb in the sense of “to take soundings,” which is to figure out the depth of water. That, in turn, led to “get to the bottom of something” in a metaphorical sense by about 1600. Which led directly to today’s current meaning. Because once you’ve gotten to the bottom of something, you understand it and comprehend it.
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Roseanna M. White is a bestselling, Christy Award winning author who has long claimed that words are the air she breathes. Having successfully launched two homeschool grads, she now spends her time writing fiction, designing book covers, and pretending her house will clean itself. Roseanna is the author of a slew of historical novels that span several continents and thousands of years, as well as a fantasy series and contemporary mysteries and romances. Spies and war and mayhem always seem to find their way into her books…to offset her real life, which is blessedly ordinary. 
