The Collaborative International Dictionary of English v.0.48:

Fathom \Fath"om\ (f[a^][th]"[u^]m), 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.
      [1913 Webster]

   2. The measure or extant of one's capacity; depth, as of
      intellect; profundity; reach; penetration. [R.]
      [1913 Webster]

            Another of his fathom they have none
            To lead their business.               --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. To measure by a sounding line; especially, to sound the
      depth of; to penetrate, measure, and comprehend; to get to
      the bottom of. --Dryden.
      [1913 Webster]

            The page of life that was spread out before me
            seemed dull and commonplace, only because I had not
            fathomed its deeper import.           --Hawthotne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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