"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.
1944, in reference to the Holocaust, probably translating German Todeslager; they also were known as extermination camps (German Vernichtungslager); historians usually count six of them: Auschwitz-Birkenau, Chełmno, Bełżec, Majdanek, Sobibór, Treblinka.
"star of David," six-pointed star, symbol of Judaism or Zionism, 1904, from Hebrew maghen Dawidh "shield of David," king of Judah and Israel, who died c. 973 B.C.E.
People are apt to sneeze, in hot weather for example; and people do not die of sneezing now-a-days, as they did in days that no one knows any thing about. We cannot give six draughts a-day, at one and nine pence each, for sneezing: call it the hay-fever. What a wonderful man! what a clever man! he understands the hay-fever: call him in! Thus is the hay-fever among the last in the list of fashionables. ["On Fashions in Physic," London Magazine, October 1825]