Jack Robinson (mythical person)

Jack Robinson is a make-believe or mythical person who is invoked in English language conversation to indicate a very short amount of time.

The normal usage is, "(something is done) faster than you can say Jack Robinson" or otherwise "… before you can say Jack Robinson."

[edit] Etymology

Several possible explanations are cited:

Supposedly, an English gentleman of the early 1800s named Jack Robinson was a person who changed his mind often, hence a person had to be quick to catch him in a decision.
Supposedly, in France in the 1800s, an umbrella was known as a Robinson, and when a gentleman needed his umbrella he would call for his servant (inevitably known as Jacques), hence, "Jacques, Robinson!"
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.

In Sam Shepard's 1967 play, Forensic & the Navigators the expression occurs late in the play. It is said by the 2nd Exterminator, before an impending gassing of the bunker.

Kiss, Kiss, Bang, Bang — early in the movie, said by the main character Harry Lockhart (played by Robert Downey Jr.)
Slap Shot — said late in the movie, by Reggie Dunlop (Paul Newman), in regards to a boy's sexuality
George Carlin: Again! — given as an example of the litany of "time"-related phrases in use in the English language