"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.
late 14c., in astrology, of two planets, "at an angular distance of 60 degrees;" 1590s (n.); from Latin sextilis (adj.) "the sixth" (in classical Latin used only in the calendar, with mensis, as the old name of August); from sextus "sixth," from sex "six" (see six).
1680s, from Latin sext-, combining form of sex "six" (see six) + ending from million. Compare billion. In English, and originally Italian, numeration, 1 to the 6th power (one followed by thirty-six zeroes); in French and U.S. use, 1,000 to the 7th power (one followed by twenty-one zeroes). Related: Sextillionth.
instrument for determining latitude in navigation and surveying, 1620s, from Modern Latin sextans, which is said to have been first used in this sense c. 1600 by Danish astronomer Tycho Brahe, from Latin sextans "a sixth, a sixth part," from sex "six" (see six). So called because the sextans has a graduated arc equal to a sixth part of a circle. In ancient Rome, sextans also was the name of a coin of the republic worth one-sixth of an as. Related: Sextantal.
1580s (n.); 1590s (adj.), in reference to Roman leap year, from Late Latin (annus) bisextilis "leap year," more literally "the twice sixth-day, (a year) containing a second sixth (day)." To keep the Julian calendar consistent with the sun, the sixth day (by inclusive reckoning) before the Calends of March was doubled every four years. The date corresponds to our February 24th. From Latin bissextus/bisextis (dies), from bis "twice" (see bis-) + sextus "sixth (day before the First of March)," from sex "six" (see six).
c. 1300 (early 13c. in Anglo-Latin), "a type of rich silk cloth," from Old French samit, from Medieval Latin samitum, examitum, from Medieval Greek hexamiton (source of Old Church Slavonic oksamitu, Russian aksamit "velvet"), noun use of neuter of Greek adjective hexamitos "six-threaded," from hex "six" (see six) + mitos "warp thread," a word of uncertain etymology.
The reason it was called this is variously explained; the traditional explanation is that it was woven of six fibers, or in a pattern involving six. Obsolete c. 1600; revived loosely by Tennyson. German Sammet "velvet" is from French.
1738, "person sixty years old or between sixty and seventy years old," from Latin sexagenarius "containing sixty," from sexagenarius, from sexageni "sixty each, sixty at a time," from sexaginta "sixty," from combining form of sex (see six). With -genaria "ten times," from -ginta "tens" (from PIE *dkm-ta-, from root *dekm- "ten"). As an adjective, "pertaining to or characteristic of the age from sixty to seventy," from 1836.