Today, fathom either serves as a word meaning "understand" or as a nautical unit of measurement equal to six feet, with both senses coming from the Old English word fæðm, meaning "embrace". The verb definition emerged from a notion of comprehension being a sort of envelopment, like a hug. The unit came about because, at one point, fæðm could also refer to a "length of outstretched arms", which is about six feet wide. Now that all that's explained, the word comes from Proto-Germanic faþmaz (also "embrace") and ultimately derives from the Proto-Indo-European root peth, meaning "to spread out". The Proto-Germanic word has an f because of a regular sound change from PIE, but some related words on the Latin side of English include expand, petal, and patent.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.