Etymology
Advertisement
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.
Related entries & more 
Advertisement
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.
Related entries & more 
unfathomed (adj.)
1620s, from un- (1) "not" + past participle of fathom (v.).
Related entries & more 
fathomable (adj.)
1630s, figurative; 1690s, literal; from fathom (v.) + -able.
Related entries & more 
fathomless (adj.)
1630s, literal ("bottomless"); 1640s, figurative ("not to be comprehended"); from fathom + -less.
Related entries & more 
Advertisement
embrace (v.)
mid-14c., "clasp in the arms," from Old French embracier (12c., Modern French embrasser) "clasp in the arms, enclose; covet, handle, cope with," from assimilated form of en- "in" (see en- (1)) + brace, braz "the arms," from Latin bracchium (neuter plural brachia) "an arm, a forearm," from Greek brakhion "an arm" (see brachio-). Related: Embraced; embracing; embraceable. Replaced Old English clyppan (see clip (v.2)), also fæðm (see fathom (v.)). Sexual sense is from 1590s.
Related entries & more 
plummet (v.)

1620s, "to fathom, take soundings," from plummet (n.). Meaning "to fall rapidly" is recorded from 1933, perhaps originally among aviators. Middle English plumben (see plumb (v.)) also meant "to plunge downward." Related: Plummeted; plummeting.

Related entries & more 
sound (v.2)
"fathom, probe, measure the depth of," mid-14c. (implied in sounding), from Old French sonder, from sonde "sounding line," perhaps from the same Germanic source that yielded Old English sund "water, sea" (see sound (n.2)). Barnhart dismisses the old theory that it is from Latin subundare. Figurative use from 1570s.
Related entries & more 
clip (v.2)

"fasten, hold together by pressure," also (mostly archaic) "to embrace," from Old English clyppan "to embrace, clasp; surround; prize, honor, cherish," from Proto-Germanic *kluppjan (source also of Old Frisian kleppa "to embrace, love," Old High German klaftra, German klafter "fathom" (on notion of outstretched arms). Also compare Lithuanian glėbys "armful," globti "to embrace."

Meaning "to fasten, bind" is early 14c. Meaning "to fasten with clips" is from 1902. Related: Clipped. Original sense of the verb is preserved in U.S. football penalty (see clipping (n.1)).

Related entries & more 
brace (n.)
early 14c., "piece of armor for the arms," also "thong, strap for fastening," from Old French brace "arms," also "length measured by two arms" (12c., Modern French bras "arm, power;" brasse "fathom, armful, breaststroke"), from Latin bracchia, plural of bracchium "an arm, a forearm," from Greek brakhion "an arm" (see brachio-).

Meaning "that which holds two or more things firmly together" (on notion of clasping arms) is from mid-15c. Hence applied to various devices for fastening and tightening. Meaning "a prop, support," especially in architecture, is from 1520s. Of dogs, ducks, pistols, etc., "a couple, a pair" from c. 1400. Braces is from 1798 as "straps passing over the shoulders to hold up the trousers;" from 1945 as "wires for straightening the teeth."
Related entries & more