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.