Mid 19th century: alteration of sestet, suggested by Latin sex 'six'.

• six from Middle English:

  The number six is Old English, but comes from the same ancient root as Latin sex and Greek hexa ‘six’. These gave us sextet (mid 19th century), sextuple (mid 16th century), hexagon (late 16th century), and similar words (compare seven). In cricket a six is a hit that sends the ball clear over to the boundary without first striking the ground, scoring six runs. The ball needs to be struck hard to go that far, and this is the image behind the expression to knock for six, ‘to utterly surprise or overcome’, recorded from the beginning of the 20th century. A form of the phrase also occurs as to hit for six, which tends to have the slightly different meaning of ‘to affect very severely’. The origins of at sixes and sevens, ‘in a state of total confusion and disarray’, lie in gambling with dice. The phrase first occurs in Geoffrey Chaucer's poem Troilus and Criseyde, in the version to set on six and seven. It is most likely that the phrase was an alteration of the Old French words for five and six, cinque and sice, these being the highest numbers on a dice. The ‘inflation’ of the numbers probably came about either because people who did not know French misheard the words, or as a jokey exaggeration. The idea was that betting on the possibility of these two numbers coming up was the height of recklessness, and could result in your whole world falling apart. A man's six-pack is his toned midriff—the abdominal muscle is crossed by three bands of fibre which look like a set of six separate muscles if the person is slim and fit. The original six-pack is associated more with couch-potatoes, as it is a pack of six cans of beer held together with a plastic fastener.