Why was a knife called a jack knife?

Answer 1

http://www.answers.com/library/American%20Word%20Origins-cid-2228396 jackknife

Origin: 1711 Americans may not have been the first to think of making a knife safe and portable by giving it a blade that folds into the handle, but we were the first to call it a jackknife. Our earliest notice is in the official records of Springfield, Massachusetts, for 1711: "One Dozen of Jack Knives, at six http://www.answers.com/topic/pence the knife" on one occasion, and "Eleven iron handled Jack knives" on another. Jackknives were a traditional accouterment for boys who were growing up in America. Without them the playing of mumble-te-peg (an English game which antedates jackknife by eighty years at least) and the whittling of sticks (which probably antedates English and perhaps even the Bronze Age) can only be practiced by the possessors of the larger and more formidable http://www.answers.com/topic/sheath knife (1837). The practical advantage to the jackknife lies in its relatively safe transportability in one's pocket. Where did we get the term? Perhaps it came from the Scottish word for a similar knife, jockteleg knife or Jock the Leg Knife, attested as early as 1672. Or perhaps it was from Jackmeaning "sailor," since the knife was used by sailors. In modern times, the figure of a jackknife opening and closing has found application beyond the tool itself. Its name has been given not only to a style of diving (1922), but to a highway accident where a tractor-trailer truck folds in on itself. * jackknife