hebdomad n.

"1 more than six; the cardinal number which is one more than six; a symbol representing this number;" Old English seofon, from Proto-Germanic *sebun (source also of Old Saxon sibun, Old Norse sjau, Swedish sju, Danish syv, Old Frisian sowen, siugun, Middle Dutch seven, Dutch zeven, Old High German sibun, German sieben, Gothic sibun), from PIE *septm "seven" (source also of Sanskrit sapta, Avestan hapta, Hittite shipta, Greek hepta, Latin septem, Old Church Slavonic sedmi, Lithuanian septyni, Old Irish secht, Welsh saith).

Long regarded as a number of perfection (seven wonders; seven sleepers, the latter translating Latin septem dormientes; seven against Thebes, etc.), but that notion is late in Old English and in German a nasty, troublesome woman could be eine böse Sieben "an evil seven" (1662). Magical power or healing skill associated since 16c. with the seventh son ["The seuenth Male Chyld by iust order (neuer a Gyrle or Wench being borne betweene)," Thomas Lupton, "A Thousand Notable Things," 1579]. The typical number for "very great, strong," as in seven-league boots in the fairy story of Hop o'my Thumb. Also, formerly, in combination with days, years, etc., indicating merely a very long time.

By early 15c. as "the hour of 7 o'clock." As a high-stakes number for a throw in dice, late 14c. The Seven Years' War (1756-63) is also the Third Silesian War. The Seven Stars (Old English sibunsterri), usually refers to the Pleiades, though in 15c. and after this name occasionally was given to the Big Dipper (which also has seven stars), or the seven planets of classical astronomy. Popular as a tavern sign, it might also (with six in a circle, one in the center) be a Masonic symbol.

FOOL: ... The reason why the seven stars are no more than seven is a pretty reason.

LEAR: Because they are not eight?

FOOL: Yes, indeed: thou wouldst make a good fool.

["King Lear," I.v.]

1610s, from Late Latin hebdomadalis, from Latin hebdomas "seven, the seventh day; a week" (see hebdomad). In later use as pedantic humor.