Fathom
Old English fæthm, of Germanic origin; related to Dutch vadem, vaam and German Faden ‘six feet’. The original sense was ‘something which embraces’, (plural) ‘the outstretched arms’; hence, a unit of measurement based on the span of the outstretched arms, later standardized to six feet.
wiktionary
From Middle English fathome, fadome, from Old English fæþm, fæþme(“outstretched or encircling arms, embrace, grasp, protection, interior, bosom, lap, breast, womb, fathom, cubit, power, expanse, surface”), from Proto-Germanic *faþmaz(“embrace”), from Proto-Indo-European *pet-(“to spread out, extend”). Cognate with Low German fadem, faem(“a cubit, thread”), Dutch vadem, vaam(“fathom”), German Faden(“thread, filament, fathom”), Danish favn(“embrace, fathom”), Norwegian Bokmål favn(“embrace, fathom”), Swedish famn(“the arms, bosom, embrace”), Icelandic faðmur(“embrace”), Latin pateō, Ancient Greek πετάννυμι(petánnumi), Ancient Greek πέταλος(pétalos) [whence English petal].
etymonline
fathom (n.)
Old English fæðm "length of the outstretched arm" (a measure of about six feet), also "arms, grasp, embrace," and, figuratively "power," from Proto-Germanic *fathmaz "embrace" (source also of Old Norse faðmr "embrace, bosom," Old Saxon fathmos "the outstretched arms," Dutch vadem "a measure of six feet"), from PIE *pot(ə)-mo-, suffixed form of root *pete- "to spread." It has apparent cognates in Old Frisian fethem, German faden "thread," which OED explains by reference to "spreading out." As a unit of measure, in an early gloss it appears for Latin passus, which was about 5 feet.
fathom (v.)
Old English fæðmian "to embrace, surround, envelop," from a Proto-Germanic verb derived from the source of fathom (n.); cognates: Old High German fademon, Old Norse faþma. The meaning "take soundings" is from c. 1600; its figurative sense of "get to the bottom of, penetrate with the mind, understand" is from 1620s. Related: Fathomed; fathoming.
