Etymology
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deep six (n.)

"place where something is discarded," by 1921 (in phrase give (something) the deep six), originally in motorboating slang, perhaps from earlier underworld noun sense of "the grave" (1929), which is perhaps a reference to the usual grave depth of six feet. But the phrase (in common with mark twain) also figured in sailing jargon, of sounding, for a measure of six fathoms:

As the water deepened under her keel the boyish voice rang out from the chains: "By the mark five—and a quarter less six—by the deep six—and a half seven—by the deep eight—and a quarter eight." ["Learning the Road to Sea," in Outing magazine, February 1918]

In general use by 1940s. As a verb from 1953.

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six (adj., n.)

one more than five; twice three; the number which is one more than five; a symbol representing this number;" Old English siex, six, seox, sex, from Proto-Germanic *seks (source also of Old Saxon and Danish seks, Old Norse, Swedish, and Old Frisian sex, Middle Dutch sesse, Dutch zes, Old High German sehs, German sechs, Gothic saihs), from PIE *s(w)eks (source also of Sanskrit sas, Avestan kshvash, Persian shash, Greek hex, Latin sex, Old Church Slavonic sesti, Polish sześć, Russian shesti, Lithuanian šeši, Old Irish se, Welsh chwech).

As "playing card with six spots or pips" by 1590s. Six-footer "person measuring six feet or more" is by 1844. Six-shooter, usually a revolver with six chambers (firing six shots in succession), is attested from 1844; six-pack of beverage containers is from 1952, of abdominal muscles by 1995.

Six of one and half-a-dozen of the other "little difference" is recorded from 1833. Six-figure (adj.) in reference to hundreds of thousands (of dollars, etc.) is from 1840. Six feet under "dead" is from 1942.

The phrase at sixes and sevens originally was "hazarding all one's chances," first in Chaucer, perhaps from dicing (the original form was on six and seven); it could be a corruption of at cinque and sice "exposed to great risk" (1530s), literally "at five and six," using the French names (which were common in Middle English) for the highest numbers on the dice. The meaning "at odds, in disagreement or confusion" in the exact phrase is from 1785; in Middle English the phrase set at (or on) six and seven meant "play havoc, create an uproar."

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deep (n.)

Old English deop "deep water," especially the sea, from the source of deep (adj.). Cognate with Old High German tiufi, German Tief, Teufe, Dutch diep, Danish dyb. General sense of "that which is of great depth" is by mid-14c.

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deep (adj.)

Old English deop "having considerable extension downward," especially as measured from the top or surface, also figuratively, "profound, awful, mysterious; serious, solemn," from Proto-Germanic *deupaz (source also of Old Saxon diop, Old Frisian diap, Dutch diep, Old High German tiof, German tief, Old Norse djupr, Danish dyb, Swedish djup, Gothic diups "deep"), from PIE root *dheub- "deep, hollow" (source also of Lithuanian dubus "deep, hollow," Old Church Slavonic duno "bottom, foundation," Welsh dwfn "deep," Old Irish domun "world," via sense development from "bottom" to "foundation" to "earth" to "world").

By early 14c. "extensive in any direction analogous to downward," as measured from the front. From late 14c. of sound, "low in pitch, grave," also of color, "intense." By c. 1200, of persons, "sagacious, of penetrating mind." From 1560s, of debt., etc., "closely involved, far advanced."

Deep pocket as figurative of wealth is from 1951. To go off the deep end "lose control of oneself" is slang recorded by 1921, probably in reference to the deep end of a swimming pool, where a person on the surface can no longer touch bottom. When 3-D films seemed destined to be the next wave and the biggest thing to hit cinema since talkies, they were known as deepies (1953)., hard to understand

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deep (adv.)

"far down, deeply," Old English deope, from the source of deep (adj.).

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knee-deep (adj.)

c. 1400, "up to the knees," from knee (n.) + deep (adj.).

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deep-seated (adj.)

1741, "having its root far below the surface," hence "firmly implanted," from deep (adv.) + past participle of seat (v.). Figurative use is by 1847. Deep-rooted is from early 15c.

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eighty-six (v.)

slang for "eliminate," 1936, originated at lunch counters, a cook's word for "none" when asked for something not available, probably rhyming slang for nix.

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deep-sea (adj.)

"of or pertaining to the deeper parts of the ocean," 1620s, from deep (adj.) + sea.

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deep-set (adj.)

"set far downward or inward," originally of eyes, late 14c., from deep (adv.) + set (adj.).

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