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]

Moby Thesaurus II by Grady Ward, 1.0:

155 Moby Thesaurus words for "fathom":
   absorb, answer, appraise, appreciate, apprehend, ascertain, assay,
   assess, assimilate, be acquainted with, be apprised of,
   be aware of, be cognizant of, be conscious of, be conversant with,
   be informed, be with one, bottom, calculate, calibrate, caliper,
   cast the lead, catch, catch on, check a parameter, clear up,
   cognize, comprehend, compute, conceive, conceptualize, crack,
   debug, decipher, decode, delve into, determine, dial, dig,
   dig into, digest, discern, disentangle, divide, divine, do, dope,
   dope out, estimate, evaluate, explain, explore, figure out,
   find out, find the answer, find the solution, follow, gauge, get,
   get hold of, get right, get the drift, get the idea,
   get the picture, go into, graduate, grasp, guess, guess right,
   have, have information about, have it, have it taped,
   have knowledge of, hit it, indagate, interpret, investigate, ken,
   know, learn, look into, make a sounding, make out, master, measure,
   mensurate, mete, meter, open the lock, pace, peer into, penetrate,
   perceive, pierce, plumb, plumb the depths, plumb-line, poke into,
   possess, prehend, prize, probe, pry into, psych, psych out,
   puzzle out, quantify, quantize, rate, ravel, ravel out, read,
   realize, recognize, resolve, riddle, savvy, search into,
   search out, see, seize, seize the meaning, sense, sift, size,
   size up, sling the lead, solve, sort out, sound, span, step,
   survey, take, take a reading, take in, take soundings, triangulate,
   understand, undo, unlock, unravel, unriddle, unscramble, untangle,
   untwist, unweave, valuate, value, weigh, work, work out, wot,
   wot of



Easton's 1897 Bible Dictionary:

Fathom
   (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.


Bouvier's Law Dictionary, Revised 6th Ed (1856):

FATHOM. A measure of length, equal to six feet. The word is probably derived
from the Teutonic word fad, which signifies the thread or yarn drawn out in
spinning to the length of the arm, before it is run upon the spindle.
Webster; Minshew. See Ell. Vide Measure.