: : Before you can say "Jack Robinson" is a way of expressing immediacy; something will be done straight away. There is one suggested origin involving the habit of an eccentric gentleman who was renowned for his constant change of mind. He often abandoned a social call and you had to be quick to catch Jack Robinson. This is the origin given in 1811.
: : The French have an even less likely version. In the old days Robinson (from Robinson Crusoe) was a popular name for an umbrella. When these umbrellas were first introduced they were highly fashionable. The story goes that the gentry, at the first sign of rain, would call their servant, inevitably named Jacques, to raise the umbrella. The call was, of course, one of "Jacques, Robinson!"
: : The reader may take or leave these offerings as they please.
: : There is a third possibility, one which I find the most acceptable. Between 1660 and 1679 the Officer Commanding the Tower of London was one Sir John Robinson. It may be that the speed of beheading with an axe, something regularly done in the Tower at that time, may be the basis, Jack being a well known form of John.

: I don't have much to add about Jack. One reference -- "Hog on Ice & Other Curious Expressions" (1948, Harper & Row) by Charles Earle Funk -- says the expression "arose during the latter part of the eighteenth century" and nobody knows who Jack Robinson was.

: IT NEVER RAINS BUT IT POURS - "One stroke of good (or ill) fortune is often followed by many other instances of luck (or misfortune) when you least expect them. The proverb dates back to the eighteenth century. In 1726, English physician John Arbuthnot (1667-1735), published a book entitled 'It Cannot Rain But It Pours.' Jonathan Swift (1667-1745) and Alexander Pope (1688-1744) collaborated on an essay entitled 'It Cannot Rain But It Pours.' The saying has been use ever since." From "Random House Dictionary of Popular Proverbs and Sayings" by Gregory Y. Titelman (Random House, New York, 1996). The saying, in a slightly different form, is the slogan for Morton Salt: ".The company developed a salt that would be free-running even in damp weather. In 1911, a little girl with an umbrella and her now-famous slogan, 'When It Rains It Pours,' were created to promote this new product in a national consumer advertising campaign. The Morton Umbrella Girl and slogan first appeared on the blue package of table salt in 1914. Throughout the years the ageless girl has changed dresses and hairstyles to stay fashionable. She was updated in 1921, 1933, 1941, 1956 and 1968. Together they have symbolized the growth and progress of the company through the years." From http://www.mortonsalt.com/consumer/about_us/history/index.htm

But I rather doubt that John "Jack" Robinson, as commanding officer of the Tower of London, 1660-79, did any beheading himself. Not only was executioner's work considered highly unsuitable for a man in his position, it was also something of a skilled profession. It wasn't easy to cleanly separate -- with one blow -- the head from the body. (Hence the invention of the guillotine, guaranteeing a clean cut and avoiding horrible death scenes where the victim was still alive even though massively wounded.) That's why, pre-guillotine, those about to be beheaded would pay money to the hooded executioner in hopes that he would take extra care to give them instant death.

But "Jack" Robinson *would* have been the person responsible for ordering the executioner to appear at a certain date and time.