We often use "fathom" to mean "understand," especially when something seems impossible to explain. Long before it described making sense of an idea though, "fathom" measured something much more tangible.
"Fathom" has a long history. As a noun, fathom dates back to Old English, where it originally meant an embrace or a grasp. It later came to mean the distance between a person's outstretched arms to the tip of one's longest finger which was eventually standardized as six feet.
By around the 1300s, "fathom" had also become a verb. At first, it meant to encircle something with your arms, as you might try to do with the trunk of a large tree. From there, it took on the sense of measuring something, particularly the depth of water with a "fathom line."
The figurative meaning of "fathom" that we're familiar with today showed up by the 1600s: To get to the bottom of something or understand it thoroughly.
Even though "fathom" can simply mean "understand," it often appears in negative contexts. You might hear someone say they can't fathom something or that something is hard to fathom. Though, positive uses do exist, such as "I can now fathom what our astronauts are doing on their missions."
The same pattern shows up with "unfathomable," a word that has seen a recent small surge in use. While fathomable is perfectly acceptable, it's less frequent. For more on that, listen to the audio above.
