Etymology
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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.
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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.
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unfathomed (adj.)
1620s, from un- (1) "not" + past participle of fathom (v.).
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fathomable (adj.)
1630s, figurative; 1690s, literal; from fathom (v.) + -able.
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fathomless (adj.)
1630s, literal ("bottomless"); 1640s, figurative ("not to be comprehended"); from fathom + -less.
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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.
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*pete- 
*petə-, Proto-Indo-European root meaning "to spread."

It forms all or part of: compass; El Paso; expand; expanse; expansion; expansive; fathom; pace (n.); paella; pan (n.); pandiculation; pas; pass; passe; passim; passacaglia; passage; passenger; passport; paten; patent; patina; petal; spandrel; spawn.

It is the hypothetical source of/evidence for its existence is provided by: Greek petannynai "to spread out," petalon "a leaf," patane "plate, dish;" Old Norse faðmr "embrace, bosom," Old English fæðm "embrace, bosom, fathom," Old Saxon fathmos "the outstretched arms."
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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.

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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.
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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)).

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