Other forms: fathoms, fathomed
To fathom something is to work out a thing that has been puzzling you. It turns up in the negative nearly every time: I cannot fathom why she said that.
A fathom is also a unit of depth at sea, six feet of it, still printed on old charts — three fathoms of water under the keel. The two senses are one idea: you measure the depth of a thing to find its bottom. Fathom out is the everyday version, as in I couldn't fathom out how the lock worked. Unfathomable means too deep or too strange to understand at all, and it gets used about grief and about stupidity in roughly equal measure.
1verbfathomTo understand something puzzling, after some effort.
“I still cannot fathom how they got the piano up those stairs.”
2noununitA unit of depth at sea, equal to six feet.
“The wreck lies in twelve fathoms of cold water.”
Too few real examples of this word to estimate which sense is commonest.
A fathom was the span of a man's outstretched arms. Old English fæþm meant exactly that — arms held wide, an embrace — and the same word served as a measure of about six feet, used at sea for sounding the depth with a weighted line. To fathom a place was to find its bottom, and the meaning slid from water to ideas.
Proto-Indo-European
*peth₂-
Proto-Indo-European
*pet-
Proto-Germanic
*faþmaz — outstretched arms, embrace; fathom (unit of measurement)
Proto-West Germanic
*faþm — outstretched arms, embrace; fathom (unit of measurement)
Old English
fæþm
Middle English
fathome
English, today
fathom
Built from
From Middle English fathome, fadom, fadme (“unit of length of about six feet; depth of six feet for nautical soundings; loosely cubit; ell”)fadam, fadame, fademe, fadim, fadmen, fadome, fadum, fadym, fathem, fathme, fathum, fedme, fethme, feþme, (Southern England) vathym, vedme, vetheym, vethym, veðme, veþum, from Old English fæþm, fæþme (“encircling or outstretched arms, bosom, embrace; envelopment; control, grasp, power; fathom (unit of measurement); cubit”)fæðm, from Proto-West Germanic faþm (“outstretched arms, embrace; fathom (unit of measurement)”), from Proto-Germanic faþmaz (“outstretched arms, embrace; fathom (unit of measurement)”), from Proto-Indo-European pet-, peth₂- (“to spread out; to fly”). 2
Etymology from Wiktionary, CC BY-SA