fathom

UK: /ˈfæðəm/ US: /ˈfæðəm/ ,(faᵺ′əm)

Inflections of 'fathom' (n):

fathoms

npl (All usages)

fathom

npl (Can be used as a collective plural—e.g. "The ship lies many fathom deep.")

WordReference Random House Learner's Dictionary of American English © 2025

fath•om /ˈfæðəm/   n., pl. -oms, (esp. when thought of as a group) -om, v. 
n. [countable]

1. Weights and Measures, Naval Terms a nautical unit of length equal to 6 feet (1.8 m).

v. [~ + object]

2. to get to the truth of;
   comprehend;
   understand:
   I couldn't fathom his motives.

fath•om•a•ble, adj. 

WordReference Random House Unabridged Dictionary of American English © 2025

fath•om  (faᵺ′əm), n., pl. fath•oms, (esp. collectively) fath•om, v. 
n. 

1. Weights and Measures, Naval Terms a unit of length equal to six feet (1.8 meters): used chiefly in nautical measurements. Abbr.: fath

v.t. 

2. Weights and Measures to measure the depth of by means of a sounding line;
   sound.
3. to penetrate to the truth of;
   comprehend;
   understand:
   to fathom someone's motives.

• bef. 900; Middle English fathme, Old English fæthm span of outstretched arms; cognate with German Faden six-foot measure, Old Norse fathmr; akin to patent

fath′om•a•ble, adj. 
fath′om•er, n. 

Collins Concise English Dictionary © HarperCollins Publishers::

fathom / ˈfæðəm/ noun

1. a unit of length equal to six feet (1.829 metres), used to measure depths of water

verb (transitive)

1. to measure the depth of, esp with a sounding line; sound
2. to penetrate (a mystery, problem, etc); discover the meaning of

Etymology: Old English fæthm; related to Old Frisian fethem outstretched arms, Old Norse fathmr embrace, Old High German fadum cubit, Latin patēre to gape

ˈfathomable adjective