Fathom

[fath -uh m] /ˈfæð əm/

noun, plural fathoms (especially collectively) fathom.
1.
a unit of length equal to six feet (1.8 meters): used chiefly in nautical measurements.
Abbreviation: fath.
verb (used with object)
2.
to measure the depth of by means of a sounding line; sound.
3.
to penetrate to the truth of; comprehend; understand:
to fathom someone’s motives.
/ˈfæðəm/
noun
1.
a unit of length equal to six feet (1.829 metres), used to measure depths of water
2.
(mining) a unit of volume usually equal to six cubic feet, used in measuring ore bodies
3.
(forestry) a unit of volume equal to six cubic feet, used for measuring timber
verb (transitive)
4.
to measure the depth of, esp with a sounding line; sound
5.
to penetrate (a mystery, problem, etc); discover the meaning of
n.

Old English fæðm “length of the outstretched arm” (a measure of about six feet), also “arms, grasp,” and, figuratively “power,” from Proto-Germanic *fathmaz “embrace” (cf. Old Norse faðmr “embrace, bosom,” Old Saxon fathmos “the outstretched arms,” Dutch vadem “a measure of six feet”), from PIE *pot(e)-mo-, from root *pete- “to spread, stretch out” (see pace (n.)). There are apparent cognates in Old Frisian fethem, German faden “thread,” which OED explains by reference to “spreading out.”
v.

Old English fæðmian “to embrace, surround, envelop;” see fathom (n.). The meaning “take soundings” is from c.1600; its figurative sense of “get to the bottom of, understand” is 1620s. Related: Fathomed; fathoming.

(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.