Source : Webster's Revised Unabridged Dictionary (1913)

Fathom \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.

Fathom \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.

Source : WordNet®

fathom
     n 1: a linear unit of measurement (equal to 6 feet) for water
          depth [syn: {fthm}]
     2: (mining) a unit of volume (equal to 6 cubic feet) used in
        measuring bodies of ore [syn: {fthm}]
     v 1: come to understand [syn: {penetrate}, {bottom}]
     2: measure the depth of (a body of water) with a sounding line
        [syn: {sound}]