Английский Этимологический словарь - fathom (n.)

- O.E. fжрm "length of the outstretched arm" (a measure of about six feet), also "arms, grasp," and, figuratively "power," from P.Gmc. *fathmaz, from PIE *pot-/*pet- denoting "stretching out." The v. meaning of "take soundings" is 1607; its figurative sense of "get to the bottom of, understand" is 1625.