Today, fathom can mean “understand” or be a nautical unit of measurement equal to six feet. Both definitions originate from the Old English word fæðm, which means “embrace”. The verb definition developed from the concept of comprehension being like a hug. The unit of measurement was established because, at one point, fæðm also referred to a “length of outstretched arms”, roughly six feet wide. The word has its roots in Proto-Germanic faþmaz (also “embrace”) and ultimately comes from the Proto-Indo-European root peth, meaning “to spread out”. The change from PIE to Proto-Germanic resulted in the addition of an f, but some related Latin-based English words include expand, petal, and patent.
