- fathom (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."
- fathom (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.