1. [n]  (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore
2. [n]  a linear unit of measurement (equal to 6 feet) for water depth
3. [v]  measure depths with a sounding line, as of a body of water
4. [v]  come to understand

1. \Fath"om\, n. [OE. fadme, fa[eth]me, AS. f[ae][eth]m
   fathom, the embracing arms; akin to OS. fa[eth]mos the
   outstretched arms, D. vadem, vaam, fathom, OHG. fadom, fadum,
   G. faden fathom, thread, Icel. fa[eth]mr fathom, Sw. famn,
   Dan. favn; cf. Gr. ?????????? to spread out, ???????
   outspread, flat, L. patere to lie open, extend. Cf. {Patent},
   {Petal}.]
   1. A measure of length, containing six feet; the space to
      which a man can extend his arms; -- used chiefly in
      measuring cables, cordage, and the depth of navigable
      water by soundings.
   
   2. The measure or extant of one's capacity; depth, as of
      intellect; profundity; reach; penetration. [R.]
   
            Another of his fathom they have none To lead their
            business.                             --Shak.
   
   

2. \Fath"om\, v. t. [imp. & p. p. {Fathomed}; p. pr. & vb.
   n. {Fathoming}.]
   1. To encompass with the arms extended or encircling; to
      measure by throwing the arms about; to span. [Obs.]
      --Purchas.
   
   2. The measure by a sounding line; especially, to sound the
      depth of; to penetrate, measure, and comprehend; to get to
      the bottom of. --Dryden.
   
            The page of life that was spread out before me
            seemed dull and commonplace, only because I had not
            fathomed its deeper import.           --Hawthotne.
   
   

(Old A.S. faethm, "bosom," or the outstretched arms), a span of six feet (Acts 27:28). Gr. orguia (from orego, "I stretch"), the distance between the extremities of both arms fully stretched out.