Word Origin & History
fathomO.E. fæðm "length of the outstretched arm" (a measure of about six feet), also "arms, grasp," and, figuratively "power," from P.Gmc. *fathmaz "embrace" (cf. O.N. faðmr "embrace, bosom," O.S. fathmos "the outstretched arms," Du. vadem "a measure of six feet"), from PIE *pot-/*pet- denoting
"stretching out" (cf. Gk. petalon "leaf," L. patere "to be open"). The verb meaning of "take soundings" is c.1600; its figurative sense of "get to the bottom of, understand" is 1620s. Related: Fathomed; fathoming.